git-describe.txt: formatting fix

A multi-line SYNOPSIS description must be marked as [verse]

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

Documentation/git-describe.txt

@@ -8,6 +8,7 @@ git-describe - Show the most recent tag that is reachable from a commit
 
 SYNOPSIS
 --------
+[verse]
 'git describe' [--all] [--tags] [--contains] [--abbrev=<n>] <committish>...
 'git describe' [--all] [--tags] [--contains] [--abbrev=<n>] --dirty[=<mark>]