Fixed grammatical error in security doc

doc/security/asset_proxy.md

@@ -7,7 +7,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 # Proxying assets **(FREE SELF)**
 
 A possible security concern when managing a public-facing GitLab instance is
-the ability to steal a users IP address by referencing images in issues and comments.
+the ability to steal a user's IP address by referencing images in issues and comments.
 
 For example, adding `![Example image](http://example.com/example.png)` to
 an issue description causes the image to be loaded from the external