git-multi-pack-index.txt: change "folder" to "directory"

We prefer "directory" over "folder" when discussing the file system
concept. In all of our documentation, these are the only spots where we
refer to the `.git` directory as a folder. Switch to "directory", and
while doing so, add backticks to the ".git" filename to set it in
monospace.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin Ågren

Documentation/git-multi-pack-index.txt

@@ -99,13 +99,13 @@ associated `.keep` file will not be selected for the batch to repack.
 EXAMPLES
 --------
 
-* Write a MIDX file for the packfiles in the current .git folder.
+* Write a MIDX file for the packfiles in the current `.git` directory.
 +
 -----------------------------------------------
 $ git multi-pack-index write
 -----------------------------------------------
 
-* Write a MIDX file for the packfiles in the current .git folder with a
+* Write a MIDX file for the packfiles in the current `.git` directory with a
 corresponding bitmap.
 +
 -------------------------------------------------------------
@@ -118,7 +118,7 @@ $ git multi-pack-index write --preferred-pack=<pack> --bitmap
 $ git multi-pack-index --object-dir <alt> write
 -----------------------------------------------
 
-* Verify the MIDX file for the packfiles in the current .git folder.
+* Verify the MIDX file for the packfiles in the current `.git` directory.
 +
 -----------------------------------------------
 $ git multi-pack-index verify