Mention the --diff flag for format --check (#104)

Just like for the action run adding annotations, the CI is the place
where you'd want extra information.

Especially information that tells you *how* to fix an issue, which is
relevant when a contributor doesn't want to / can't run the tool locally
(an edit through the GitHub UI could be more convenient, don't want to
bother installing extra tools locally, they're having issues not picking
up the right Ruff version when ran locally, etc.)

It's also easier to opt-out of copy-pasting this flag, than not knowing
about it in the first place.

Hence I think this documentation change is a positive to help anyone
integrate this action into their CI for check+formatting by looking at
the readme.

README.md

@@ -85,7 +85,7 @@ you can use the `args` input to overwrite the default value (`check`):
 ```yaml
 - uses: astral-sh/ruff-action@v3
   with:
-    args: "format --check"
+    args: "format --check --diff"
 ```
 
 ### Install specific versions