Remove unnecessary backslash (go-gitea#8249)

Author: jolheiser

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -60,7 +60,7 @@ the url `http://gitea.domain.tld/image.png`.
 
 ## Changing the default avatar
 
-Place the png image at the following path: `custom/public/img/avatar\_default.png`
+Place the png image at the following path: `custom/public/img/avatar_default.png`
 
 ## Customizing Gitea pages