Trac #34558: Typo in git-trac file of developer's guide

There is a missing double colon, so the first code block is not rendered
correctly: look at
https://doc.sagemath.org/html/en/developer/git_trac.html and the block
at the top of the section "Installing the Git-Trac Command".

URL: https://trac.sagemath.org/34558
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Matthias Koeppe

Author: Release Manager

src/doc/en/developer/git_trac.rst

@@ -17,6 +17,8 @@ perform every development task with just git and a web browser.
 Installing the Git-Trac Command
 ===============================
 
+::
+
     [user@localhost]$ git clone https://github.com/sagemath/git-trac-command.git
     Cloning into 'git-trac-command'...
     [...]