Fixed typo

Fixes gitlab-org/gitlab-ce#27674

changelogs/unreleased/slash-commands-typo.yml

+---
+title: Fixed "substract" typo on /help/user/project/slash_commands
+merge_request: 8976
+author: Jason Aquino

doc/user/project/slash_commands.md

@@ -32,6 +32,6 @@ do.
 | `/wip`                     | Toggle the Work In Progress status |
 | <code>/estimate &lt;1w 3d 2h 14m&gt;</code> | Set time estimate |
 | `/remove_estimate`       | Remove estimated time |
-| <code>/spend &lt;1h 30m &#124; -1h 5m&gt;</code> | Add or substract spent time |
+| <code>/spend &lt;1h 30m &#124; -1h 5m&gt;</code> | Add or subtract spent time |
 | `/remove_time_spent`       | Remove time spent |
 | `/target_branch <Branch Name>` | Set target branch for current merge request |