doc: update default PowerPC framebuffer settings

Since 1.7, the default framebuffer settings for PowerPC are 800x600x32.
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
CC: qemu-ppc@nongnu.org
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-doc.texi

@@ -1938,7 +1938,7 @@ The following options are specific to the PowerPC emulation:
 
 @item -g @var{W}x@var{H}[x@var{DEPTH}]
 
-Set the initial VGA graphic mode. The default is 800x600x15.
+Set the initial VGA graphic mode. The default is 800x600x32.
 
 @item -prom-env @var{string}