gitweb: fix typo in man page

Signed-off-by: Tony Finch <dot@dotat.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitweb.conf.txt

@@ -487,7 +487,7 @@ By default it is set to (), i.e. an empty list.  This means that gitweb
 would not try to create project URL (to fetch) from project name.
 
 $projects_list_group_categories::
-	Whether to enables the grouping of projects by category on the project
+	Whether to enable the grouping of projects by category on the project
 	list page. The category of a project is determined by the
 	`$GIT_DIR/category` file or the `gitweb.category` variable in each
 	repository's configuration.  Disabled by default (set to 0).