git-filter-repo.txt: fix typesetting of --partial

Signed-off-by: Elijah Newren <[email protected]>

Author: newren

Documentation/git-filter-repo.txt

@@ -279,7 +279,7 @@ Miscellaneous options
 	Rewrite history even if the current repo does not look like a fresh
 	clone.
 
---partial:
+--partial::
 	Do a partial history rewrite, resulting in the mixture of old and
 	new history.  This implies a default of update-no-add for
 	--replace-refs, disables rewriting refs/remotes/origin/* to