doc_src: Continue the slog through the letter F.

We are using only :: in a synopsis for fishscript examples given
of the command being documented.

doc_src/cmds/fish_git_prompt.rst

@@ -6,12 +6,14 @@ fish_git_prompt - output git information for use in a prompt
 Synopsis
 --------
 
+``fish_git_prompt``
 ::
 
      function fish_prompt
           printf '%s' $PWD (fish_git_prompt) ' $ '
      end
 
+
 Description
 -----------