BB2-3529: Doc updates for PKCE (Part 2) (#393)

* Doc updates for PKCE

* Fix code_verifier example

_pages/developers.md

@@ -412,7 +412,7 @@ To retrieve an access token, POST to the BB2.0 /token endpoint providing the cod
 ~~~
 curl -X POST "https://sandbox.bluebutton.cms.gov/v2/o/token/" \
 -u "<client_id>:<client_secret>" \
--d "code=TSjqiZCdJwGyytGjz2GzziPfHTJ6z2&grant_type=authorization_code&redirect_uri=http://localhost:8080/testclient/callback"
+-d "code=TSjqiZCdJwGyytGjz2GzziPfHTJ6z2&grant_type=authorization_code&redirect_uri=http://localhost:8080/testclient/callback&code_verifier=zlGzSLRQz6HrTpd3TvEraYoVPW2cknzu4tUk6wHaPFw"
 ~~~
 ##### Token response
 ~~~