chore: fix new clippy rule empty_line_after_doc_comments

[yukiiiteru]

error: empty line after doc comment
  --> volo-thrift/src/codec/default/thrift.rs:37:5
   |
37 | /     /// This is unimplemented yet.
38 | |     // pub fn with_multiplex(mut self, multiplex: bool) -> Self {
39 | |     //     self.multiplex = multiplex;
40 | |     //     self
41 | |     // }
42 | |
   | |_
...
45 |       pub fn with_protocol(mut self, protocol: Protocol) -> Self {
   |       ---------------------------------------------------------- the comment documents this method
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
   = note: `-D clippy::empty-line-after-doc-comments` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::empty_line_after_doc_comments)]`
   = help: if the empty line is unintentional remove it
help: if the doc comment should not document `with_protocol` comment it out
   |
29 ~     // /// Whether to use thrift multiplex protocol.
30 ~     // ///
31 ~     // /// When the multiplexed protocol is used, the name contains the service name,
32 ~     // /// a colon : and the method name. The multiplexed protocol is not compatible
33 ~     // /// with other protocols.
34 ~     // ///
35 ~     // /// Spec: <https://github.com/apache/thrift/blob/master/doc/specs/thrift-rpc.md>
36 ~     // ///
37 ~     // /// This is unimplemented yet.
   |

error: could not compile `volo-thrift` (lib) due to 1 previous error

[yukiiiteru]

Currently, all errors are from rust-lang/rust-clippy#13458, which is a bug in clippy and can be ignored for now.