schemes: fixed typos in module docstring

hgext/schemes.py

@@ -35,8 +35,8 b' For convenience, the extension adds thes'
   bb+ssh = ssh://[e-mail]/
   gcode = https://{1}.googlecode.com/hg/
 
-You can override the predefined scheme by defining new schemes with
-the same name.
+You can override a predefined scheme by defining a new scheme with the
+same name.
 """
 
 import re