Stub out Appendix C for linking

This is to make sure PR checks pass even though the PR adding
Appendix C has not yet been merged.

versions/3.0.4.md

@@ -3531,6 +3531,8 @@ Version   | Date       | Notes
 1.1       | 2012-08-22 | Release of Swagger 1.1
 1.0       | 2011-08-10 | First release of the Swagger Specification
 
+## <a name="usingRFC6570Implementations"></a>Appendix C: Using RFC6570 Implementations
+
 ## <a name="percentEncodingAndFormMediaTypes"></a>Appendix E: Percent-Encoding and Form Media Types
 
 _**NOTE:** In this section, the `application/x-www-form-urlencoded` and `multipart/form-data` media types are abbreviated as `form-urlencoded` and `form-data`, respectively, for readability._
@@ -3552,7 +3554,7 @@ Reserved characters with no special meaning defined within a component can be le
 However, other specifications can define special meanings, requiring percent-encoding for those characters outside of the additional special meanings.
 
 The `form-urlencoded` media type defines special meanings for `=` and `&` as delimiters, and `+` as the replacement for the space character (instead of its percent-encoded form of `%20`).
-This means that while these three characters are reserved-but-allowed in query strings by RFC3986, the must be percent-encoded in `form-urlencoded` query strings except when used for their `form-urlencoded` purposes; see Appendix C for an example of handling `+` in form values.
+This means that while these three characters are reserved-but-allowed in query strings by RFC3986, the must be percent-encoded in `form-urlencoded` query strings except when used for their `form-urlencoded` purposes; see [Appendix C](#usingRFC6570Implementations) for an example of handling `+` in form values.
 
 ### Percent-Encoding and `form-data`
 
@@ -3576,7 +3578,7 @@ Specification | Date | OAS Usage | Percent-Encoding | Notes
 [RFC1866&nbsp;§8.2.1 form&#8209;urlencoded](https://datatracker.ietf.org/doc/html/rfc1866#section-8.2.1) | 11/1995 | content-based serialization | RFC1738 | obsoleted by [HTML&nbsp;4.01&nbsp;§17.13.4.1](https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1), [WHATWG&nbsp;URL&nbsp;§5](https://url.spec.whatwg.org/#urlencoded-serializing)
 
 Style-based serialization is used in the [Parameter Object](#parameterObject) when `schema` is present, and in the [Encoding Object](#encodingObject) when at least one of `style`, `explode`, or `allowReserved` is present.
-See [Appendix C]() for more details of RFC6570's two different approaches to percent-encoding, including an example involving `+`.
+See [Appendix C](#usingRFC6570Implementations) for more details of RFC6570's two different approaches to percent-encoding, including an example involving `+`.
 
 Content-based serialization is defined by the [Media Type Object](#mediaTypeObject), and used with the [Parameter Object](#parameterObject) when the `content` field is present, and with the [Encoding Object](#encodingObject) based on the `contentType` field when the style fields listed in the previous section are absent.
 Each part is encoded based on the media type (e.g. `text/plain` or `application/json`), and must then be percent-encoded for use in a `form-urlencoded` string.