color: use reST syntax for literal block

hgext/color.py

@@ -28,7 +28,7 b' available. Effects are rendered with the'
 ANSI escape codes). This module also provides the render_text function, which
 can be used to add effects to any text.
 
-Default effects may be overridden from the .hgrc file:
+Default effects may be overridden from the .hgrc file::
 
 [color]
 status.modified = blue bold underline red_background