Fix typo in doc/api/groups.md

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6123.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/api/groups.md

@@ -82,7 +82,7 @@ You can filter by [custom attributes](custom_attributes.md) with:
 GET /groups?custom_attributes[key]=value&custom_attributes[other_key]=other_value
 ```
 
-## List a groups's subgroups
+## List a group's subgroups
 
 > [Introduced][ce-15142] in GitLab 10.3.