Documentation: normalize spelling of 'normalised'

This could be a localization issue, but we had about four dozen
"normalize"s (or variants, e.g. normalized, renormalize, etc.), and only
one "normalised" (no other variants), so normalize normalised into
normalized.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mktree.txt

@@ -14,7 +14,7 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Reads standard input in non-recursive `ls-tree` output format, and creates
-a tree object.  The order of the tree entries is normalised by mktree so
+a tree object.  The order of the tree entries is normalized by mktree so
 pre-sorting the input is not required.  The object name of the tree object
 built is written to the standard output.