MAINTAINERS: Fix documentation file name for PHY Library

MAINTAINERS still pointed to phy.txt after moving this file into the rst
format, fix this.
Reported-by: Joe Perches <joe@perches.com>
Fixes: 25fe02d0 ("Documentation: net: phy: switch documentation to rst format")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -5833,7 +5833,7 @@ L:	netdev@vger.kernel.org
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-mdio
 F:	Documentation/devicetree/bindings/net/mdio*
-F:	Documentation/networking/phy.txt
+F:	Documentation/networking/phy.rst
 F:	drivers/net/phy/
 F:	drivers/of/of_mdio.c
 F:	drivers/of/of_net.c