pre-commit.sample: add comment re tr portability; fix grammar

Add a comment explaining why square brackets around a tr range
are not only ok, but actually required in this case.
Correct spelling and grammar.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-commit.sample

@@ -14,15 +14,18 @@ allownonascii=$(git config hooks.allownonascii)
 # them from being added to the repository. We exploit the fact that the
 # printable range starts at the space character and ends with tilde.
 if [ "$allownonascii" != "true" ] &&
+	# Note that the use of brackets around a tr range is ok here, (it's
+	# even required, for portability to Solaris 10's /usr/bin/tr), since
+	# the square bracket bytes happen to fall in the designated range.
 	test "$(git diff --cached --name-only --diff-filter=A -z |
 	  LC_ALL=C tr -d '[ -~]\0')"
 then
-	echo "Error: Attempt to add a non-ascii filename."
+	echo "Error: Attempt to add a non-ascii file name."
 	echo
-	echo "This can cause problems if you want to work together"
-	echo "with people on other platforms than you."
+	echo "This can cause problems if you want to work"
+	echo "with people on other platforms."
 	echo
-	echo "To be portable it is adviseable to rename the file ..."
+	echo "To be portable it is advisable to rename the file ..."
 	echo
 	echo "If you know what you are doing you can disable this"
 	echo "check using:"