maintainers: Fix PowerPC 4xx git tree

Update the git tree URL for the PowerPC 4xx git tree.
Reported-by: Denis ChengRq <crquan@gmail.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>

MAINTAINERS

@@ -3375,7 +3375,7 @@ P:	Matt Porter
 M:	mporter@kernel.crashing.org
 W:	http://www.penguinppc.org/
 L:	linuxppc-dev@ozlabs.org
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/powerpc.git
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/powerpc-4xx.git
 S:	Maintained
 
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