stmmac.txt: fix typo

Fix typo in devicetree example.
Signed-off-by: NPavel Machek <pavel@denx.de>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -54,5 +54,5 @@ Examples:
 		snps,multicast-filter-bins = <256>;
 		snps,perfect-filter-entries = <128>;
 		clocks = <&clock>;
-		clock-names = "stmmaceth">;
+		clock-names = "stmmaceth";
 	};