Fix typo on --http-api-update environment variable and add warning note for --http-api-periodic-polls (#1195)

* Update arguments.md

Fix a typo for the environment variable to enable HTTP API mode.

* Add note about http-api-periodic-polls

--http-api-periodic-polls is a new option which isn't available currently, until a new release is made.

* Update docs/arguments.md

Co-authored-by: nils måsén <nils@piksel.se>

Co-authored-by: Simon Aronsson <simme@arcticbit.se>
Co-authored-by: nils måsén <nils@piksel.se>

docs/arguments.md

@@ -241,7 +241,7 @@ For details see [HTTP API](https://containrrr.dev/watchtower/http-api-mode).
 
 ```text
             Argument: --http-api-update
-Environment Variable: WATCHTOWER_HTTP_API
+Environment Variable: WATCHTOWER_HTTP_API_UPDATE
                 Type: Boolean
              Default: false
 ```