docs: provide example of having a --option in [cli.alias] (#20539)

I have learnt only recently that you can provide a flag like `--flag` in
`[cli.alias]` and I think it would be useful to have it in the docs
among the examples.

---------

Co-authored-by: Andreas Stenius <[email protected]>

Author: AlexTereshenkov

src/python/pants/option/alias.py

@@ -46,10 +46,10 @@ class CliOptions(Subsystem):
 
                 [cli.alias]
                 green = "fmt lint check"
-                all-changed = "--changed-since=HEAD --changed-dependents=transitive"
+                --all-changed = "--changed-since=HEAD --changed-dependents=transitive"
 
 
-            This would allow you to run `{bin_name()} green all-changed`, which is shorthand for
+            This would allow you to run `{bin_name()} green --all-changed`, which is shorthand for
             `{bin_name()} fmt lint check --changed-since=HEAD --changed-dependents=transitive`.
 
             Notice: this option must be placed in a config file (e.g. `pants.toml` or `pantsrc`)