fix emacs indenting howto filename expansion

I don't think emacs understands tilde expansion, so use
"expand-file-name" to do that.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/CodingStyle

@@ -498,7 +498,8 @@ values.  To do the latter, you can stick the following in your .emacs file:
             (let ((filename (buffer-file-name)))
               ;; Enable kernel mode for the appropriate files
               (when (and filename
-                         (string-match "~/src/linux-trees" filename))
+                         (string-match (expand-file-name "~/src/linux-trees")
+                                       filename))
                 (setq indent-tabs-mode t)
                 (c-set-style "linux-tabs-only")))))