docs: Grammatical fix in stability sentence (#86)

Signed-off-by: Brandon Philips <[email protected]>

README.md

@@ -24,7 +24,7 @@ by administartive users. ORY Oathkeeper is thus capable of:
 * Restricting access to certain resources based on a set of rules (authorization).
 * Transforms access credentials (e.g. OAuth2 Access Tokens, SAML Assertions, ...) to a format (e.g. JSON Web Token, Plaintext, Basic Authorization, ...) consumable by your API services.
 
-This service stable, but under active development and may introduce breaking changes in future releases. Any breaking change will have extensive documentation and upgrade instructions.
+This service is stable, but under active development and may introduce breaking changes in future releases. Any breaking change will have extensive documentation and upgrade instructions.
 
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