doc: fix location of 'info/' with $GIT_COMMON_DIR

With the introduction of the $GIT_COMMON_DIR variable, the
repository layout manual was changed to reflect the location for
many files in case the variable is set. While adding the new
locations, one typo snuck in regarding the location of the
'info/' folder, which is falsely claimed to reside at
"$GIT_COMMON_DIR/index".

Fix the typo to point to "$GIT_COMMON_DIR/info/" instead.
Signed-off-by: NPatrick Steinhardt <ps@pks.im>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitrepository-layout.txt

@@ -177,7 +177,7 @@ sharedindex.<SHA-1>::
 info::
 	Additional information about the repository is recorded
 	in this directory. This directory is ignored if $GIT_COMMON_DIR
-	is set and "$GIT_COMMON_DIR/index" will be used instead.
+	is set and "$GIT_COMMON_DIR/info" will be used instead.
 
 info/refs::
 	This file helps dumb transports discover what refs are