Show full-path refresh() in failure message differently

This presents it more symbolically, rather than in an example-like
style that could confuse. See discussion in gitpython-developers#1844.

git/cmd.py

@@ -458,7 +458,7 @@ def refresh(cls, path: Union[None, PathLike] = None) -> bool:
                 The git executable must be specified in one of the following ways:
                     - be included in your $PATH
                     - be set via $%s
-                    - explicitly set via git.refresh("/full/path/to/git")
+                    - explicitly set via git.refresh(<full-path-to-git-executable>)
                 """
                 )
                 % cls._git_exec_env_var