help: use 'color' as an example (instead of 'progress')

Progress is now deprecated, using it as an example is suboptimal.

mercurial/help/config.txt

@@ -641,8 +641,8 b' or ``foo = !`` when path is not supplied'
 Example for ``~/.hgrc``::
 
   [extensions]
-  # (the progress extension will get loaded from Mercurial's path)
-  progress =
+  # (the color extension will get loaded from Mercurial's path)
+  color =
   # (this extension will get loaded from the file specified)
   myfeature = ~/.hgext/myfeature.py