Documentation: fix misrender of pretty-formats in Asciidoctor

Neither the AsciiDoc nor the Asciidoctor documentation specify whether
the same number of delimiter characters must be used to end a block as
to begin it, although both sets of documentation show exactly matching
pairs.  AsciiDoc allows mismatches, but AsciiDoctor apparently does not.
Adjust the pretty formats documentation to use matching pairs to prevent
a misrendering where the remainder of the document was rendered as a
listing block.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -90,7 +90,7 @@ would show something like this:
 The author of fe6e0ee was Junio C Hamano, 23 hours ago
 The title was >>t4119: test autocomputing -p<n> for traditional diff input.<<
 
---------
+-------
 +
 The placeholders are: