fix: psk -> key (#220)

Author: aquamatthias

docs/reference/security.mdx

@@ -118,7 +118,7 @@ The following will return http headers that contain a valid <abbr title="JSON we
 
 ```python
 >>> from fixlib.jwt import encode_jwt_to_headers
->>> encode_jwt_to_headers(http_headers={}, payload={}, psk="changeme", expire_in=3600)
+>>> encode_jwt_to_headers(http_headers={}, payload={}, key="changeme", expire_in=3600)
 # highlight-next-line
 ​{'Authorization': 'Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiIsInNhbHQiOiJuSVEzU3M5TGVNS1JHYUNQUEJxMnlBPT0ifQ.eyJleHAiOjE2NDkzNzI1MTR9.KXAmijfSsV-taO3890qJNzXKXng1u38eU6PTrDYTgVs'}
 ```

versioned_docs/version-4.X/reference/security.mdx

@@ -118,7 +118,7 @@ The following will return http headers that contain a valid <abbr title="JSON we
 
 ```python
 >>> from fixlib.jwt import encode_jwt_to_headers
->>> encode_jwt_to_headers(http_headers={}, payload={}, psk="changeme", expire_in=3600)
+>>> encode_jwt_to_headers(http_headers={}, payload={}, key="changeme", expire_in=3600)
 # highlight-next-line
 ​{'Authorization': 'Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiIsInNhbHQiOiJuSVEzU3M5TGVNS1JHYUNQUEJxMnlBPT0ifQ.eyJleHAiOjE2NDkzNzI1MTR9.KXAmijfSsV-taO3890qJNzXKXng1u38eU6PTrDYTgVs'}
 ```