docs: Replace A icon with help in format using markdown.

In the format-your-message-using-markdown, in the in-help
help section, the A icon is replace by help button. This
updates the docs.

templates/zerver/help/format-your-message-using-markdown.md

@@ -273,7 +273,7 @@ A summary of the formatting syntax is available in-app.
 
 {!start-composing.md!}
 
-1. Click the A (<i class="fa fa-font"></i>) icon at the bottom of the compose box.
+1. Click help at the bottom of the compose box.
 
 {end_tabs}