gitignore.txt: change "folder" to "directory"

We prefer "directory" over "folder" when discussing the file system
concept. Change this instance for consistency -- indeed, even within
this paragraph, we already use "directory".

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

Author: Martin Ågren

Documentation/gitignore.txt

@@ -155,7 +155,7 @@ accessed from the index or a tree versus from the filesystem.
 EXAMPLES
 --------
 
- - The pattern `hello.*` matches any file or folder
+ - The pattern `hello.*` matches any file or directory
    whose name begins with `hello.`. If one wants to restrict
    this only to the directory and not in its subdirectories,
    one can prepend the pattern with a slash, i.e. `/hello.*`;