Fix formatting of 'repo help sync'

The formatting for the enviroment variable section was incorrect.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/forall.py

@@ -30,6 +30,7 @@ Executes the same shell command in each project.
 
 Environment
 -----------
+
 pwd is the project's working directory.  If the current client is
 a mirror client, then pwd is the Git repository.