Fix typo in toplevel signal name

I must have screwed this up when adding the PLL. It's surprising
that yosys didn't complain.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

toplevel.v

@@ -8,7 +8,7 @@ module toplevel(
 	output io_ledC
 );
 
-wire clk_out;
+wire clock_out;
 wire reset_out;
 wire lock;