Doc add: link gitignore

Use a gitignore link rather than the gitrepository-
layout link.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -155,7 +155,7 @@ Configuration
 The optional configuration variable `core.excludesfile` indicates a path to a
 file containing patterns of file names to exclude from git-add, similar to
 $GIT_DIR/info/exclude.  Patterns in the exclude file are used in addition to
-those in info/exclude.  See linkgit:gitrepository-layout[5].
+those in info/exclude.  See linkgit:gitignore[5].
 
 
 EXAMPLES