docs: typos and minor clarifications to oauth2 http filter (#35581)

Signed-off-by: Dean Liu <dliu@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ b5bbfba79379eaf4824c3a96c4d9d19bd498a31a

envoy/extensions/filters/http/oauth2/v3/oauth.proto

@@ -153,7 +153,7 @@ message OAuth2Config {
   //
   // If this value is not set, it will default to ``604800s``. In this case, the cookie with the refresh token will be expired
   // in a week.
-  // This setting is only considered if ``use_refresh_token`` is set to true, otherwise the authorization server expiration or ``defaul_expires_in`` is used.
+  // This setting is only considered if ``use_refresh_token`` is set to true, otherwise the authorization server expiration or ``default_expires_in`` is used.
   google.protobuf.Duration default_refresh_token_expires_in = 15;
 
   // If set to true, Envoy will not set a cookie for ID Token even if one is received from the Identity Provider. This may be useful in cases where the ID