runtime: fix typo in error message

LGTM=robert.hencke, iant
R=golang-codereviews, robert.hencke, iant
CC=golang-codereviews
https://golang.org/cl/89760043
diff --git a/src/pkg/runtime/malloc.goc b/src/pkg/runtime/malloc.goc
index 6fee893..6371689 100644
--- a/src/pkg/runtime/malloc.goc
+++ b/src/pkg/runtime/malloc.goc
@@ -684,7 +684,7 @@
 
 	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