Documentation/Makefile: make manpage-base-url.xsl generation quieter

Documentation/Makefile: make manpage-base-url.xsl generation quieter

The exact sed command to generate manpage-base-url.xsl appears in
the output, unlike the rules for other files that by default only
show summary.

Make the output for this rule similiar to all the other rules by
printing a short status message instead of the whole command.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>