Documentation: show-files is now called git-ls-files

Amazingly, a reference to 'show files' survived from the core command
documentation introduced in c64b9b8 (Reference documentation for the
core git commands., 2005-05-05)!

However, the tool is now called git-ls-files.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>

Documentation/git-ls-files.txt

@@ -140,7 +140,7 @@ OPTIONS
 
 Output
 ------
-show files just outputs the filename unless '--stage' is specified in
+'git ls-files' just outputs the filenames unless '--stage' is specified in
 which case it outputs:
 
         [<tag> ]<mode> <object> <stage> <file>