extdiff: grammar "allows to" -> "allows one to"

The verb to allow requires a direct object.

Lintian, a Debian tool to find common mistakes, reported it. I'm not a
native english speaker but I think this is correct.

hgext/extdiff.py

@@ -13,7 +13,7 b' diff programs are called with a configur'
 non-option arguments: paths to directories containing snapshots of
 files to compare.
 
-The extdiff extension also allows to configure new diff commands, so
+The extdiff extension also allows you to configure new diff commands, so
 you do not need to type :hg:`extdiff -p kdiff3` always. ::
 
   [extdiff]

tests/test-extension.t

@@ -277,8 +277,8 b' Extension module help vs command help:'
   called with a configurable set of options and two non-option arguments: paths
   to directories containing snapshots of files to compare.
   
-  The extdiff extension also allows to configure new diff commands, so you do
-  not need to type "hg extdiff -p kdiff3" always.
+  The extdiff extension also allows you to configure new diff commands, so you
+  do not need to type "hg extdiff -p kdiff3" always.
   
     [extdiff]
     # add new command that runs GNU diff(1) in 'context diff' mode