Documentation: remove --prune from pack-refs examples

The option has been the default for a while, and doesn't otherwise
appear in the page.

Signed-off-by: Jonathon Mah <me@JonathonMah.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-refs.txt

@@ -33,8 +33,8 @@ Subsequent updates to branches always create new files under
 `$GIT_DIR/refs` directory hierarchy.
 
 A recommended practice to deal with a repository with too many
-refs is to pack its refs with `--all --prune` once, and
-occasionally run `git pack-refs --prune`.  Tags are by
+refs is to pack its refs with `--all` once, and
+occasionally run `git pack-refs`.  Tags are by
 definition stationary and are not expected to change.  Branch
 heads will be packed with the initial `pack-refs --all`, but
 only the currently active branch heads will become unpacked,