Merge branch 'patch-3' into 'master'

Changed grant_type=AUTHORIZATION_CODE for grant_type=authorization_code

## What does this MR do?

Changed grant_type=AUTHORIZATION_CODE for grant_type=authorization_code 

## Why was this MR needed?
Using the example when provided the parameter grant_type=AUTHORIZATION_CODE the server returns a error:
```json 
{"error":"unsupported_grant_type","error_description":"The authorization grant type is not supported by the authorization server."}
```


See merge request !5539

doc/api/oauth2.md

@@ -35,7 +35,7 @@ Where REDIRECT_URI is the URL in your app where users will be sent after authori
 To request the access token, you should use the returned code and exchange it for an access token. To do that you can use any HTTP client. In this case, I used rest-client:
 
 ```
-parameters = 'client_id=APP_ID&client_secret=APP_SECRET&code=RETURNED_CODE&grant_type=AUTHORIZATION_CODE&redirect_uri=REDIRECT_URI'
+parameters = 'client_id=APP_ID&client_secret=APP_SECRET&code=RETURNED_CODE&grant_type=authorization_code&redirect_uri=REDIRECT_URI'
 RestClient.post 'http://localhost:3000/oauth/token', parameters
 
 # The response will be