📝 Fix four-backtick code fence in OAuth2PasswordRequestForm docstrings

Both OAuth2PasswordRequestForm and OAuth2PasswordRequestFormStrict had a
closing ```` ``` ```` with four backticks instead of the standard three.
The mismatch left a stray backtick in generated reference docs (e.g.
`help()` output and any doc-builder that parses the docstring).

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

fastapi/security/oauth2.py

@@ -112,7 +112,7 @@ class OAuth2PasswordRequestForm:
 
                 ```python
                 "items:read items:write users:read profile openid"
-                ````
+                ```
 
                 would represent the scopes:
 
@@ -278,7 +278,7 @@ class OAuth2PasswordRequestFormStrict(OAuth2PasswordRequestForm):
 
                 ```python
                 "items:read items:write users:read profile openid"
-                ````
+                ```
 
                 would represent the scopes: