Add an example of --head option usage to pr list docs #10957

Description

Related to #10945

As a small improvement, another example should be added to pr list docs to show how the --head option can be used. Something like this:

EXAMPLES
  ...

  # List PRs with a specific head branch name
  $ gh pr list --head "typo"

The reason I'm suggesting this is that the --head option does not work with values formatted as <owner>:<branch>. It's actually our API's limit. We cannot explicitly say this in gh docs, because it's an undocumented behaviour which may change in the future; especially since branch names cannot include :, supporting this new format in the API will not be a breaking change.

Acceptance Criteria

When I run gh pr list --help
Then the docs include an example of the --head option usage