MAINTAINERS: update file pattern for ARC uart

Commit 6659a20 ("ARC: MAINTAINERS update for ARC") typoed the file
pattern.  Fix it.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -8082,7 +8082,7 @@ M:	Vineet Gupta <vgupta@synopsys.com>
 S:	Supported
 F:	arch/arc/
 F:	Documentation/devicetree/bindings/arc/
-F:	drivers/tty/serial/arc-uart.c
+F:	drivers/tty/serial/arc_uart.c
 
 SYSV FILESYSTEM
 M:	Christoph Hellwig <hch@infradead.org>