runtime: fix typo in error message

LGTM=robert.hencke, iant
R=golang-codereviews, robert.hencke, iant
CC=golang-codereviews
https://golang.org/cl/89760043

src/pkg/runtime/malloc.goc

@@ -684,7 +684,7 @@ runtime·persistentalloc(uintptr size, uintptr align, uint64 *stat)
 
 	if(align != 0) {
 		if(align&(align-1))
-			runtime·throw("persistentalloc: align is now a power of 2");
+			runtime·throw("persistentalloc: align is not a power of 2");
 		if(align > PageSize)
 			runtime·throw("persistentalloc: align is too large");
 	} else