Fix typo in authentication section of README (modelcontextprotocol#676)

Author: mopg

README.md

@@ -318,7 +318,7 @@ providing an implementation of the `OAuthServerProvider` protocol.
 
 ```
 mcp = FastMCP("My App",
-        auth_provider=MyOAuthServerProvider(),
+        auth_server_provider=MyOAuthServerProvider(),
         auth=AuthSettings(
             issuer_url="https://myapp.com",
             revocation_options=RevocationOptions(