doc: mention 'git show' defaults to HEAD

When 'git show' is called without any object it defaults to HEAD.  This
has been true since d4ed9793 ("Simplify common default options setup
for built-in log family.", 2006-04-16).

The SYNOPSIS suggests that the object argument is required.  Clarify
that it is not required and note the default.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show.txt

@@ -9,7 +9,7 @@ git-show - Show various types of objects
 SYNOPSIS
 --------
 [verse]
-'git show' [options] <object>...
+'git show' [options] [<object>...]
 DESCRIPTION
 -----------
@@ -35,7 +35,7 @@ This manual page describes only the most frequently used options.
 OPTIONS
 -------
 <object>...::
-	The names of objects to show.
+	The names of objects to show (defaults to 'HEAD').
 	For a more complete list of ways to spell object names, see
 	"SPECIFYING REVISIONS" section in linkgit:gitrevisions[7].