MAINTAINERS: correct at91 tree link

This was not done when the tree name was changed, fix it now.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 228d8af..3bf60c4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -408,7 +408,7 @@
 ARM MICROCHIP/ATMEL AT91
 M:	Eugen Hristev <eugen.hristev@microchip.com>
 S:	Maintained
-T:	git https://source.denx.de/u-boot/custodians/u-boot-atmel.git
+T:	git https://source.denx.de/u-boot/custodians/u-boot-at91.git
 F:	arch/arm/mach-at91/
 F:	board/atmel/
 F:	drivers/cpu/at91_cpu.c