Update JWT docs in example config (#12591)

* Update JWT docs in example config

align with way we have `LFS_JWT_SECRET` in config

Fix #12590

* Update custom/conf/app.example.ini

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

custom/conf/app.example.ini

@@ -971,8 +971,8 @@ ACCESS_TOKEN_EXPIRATION_TIME=3600
 REFRESH_TOKEN_EXPIRATION_TIME=730
 ; Check if refresh token got already used
 INVALIDATE_REFRESH_TOKENS=false
-; OAuth2 authentication secret for access and refresh tokens, change this to a unique string.
+; OAuth2 authentication secret for access and refresh tokens, change this yourself to a unique string. CLI generate option is helpful in this case. https://docs.gitea.io/en-us/command-line/#generate
-JWT_SECRET=Bk0yK7Y9g_p56v86KaHqjSbxvNvu3SbKoOdOt2ZcXvU
+JWT_SECRET=
 ; Maximum length of oauth2 token/cookie stored on server
 MAX_TOKEN_LENGTH=32767