Fix the error message when .create_stats() is called outside vcl_init.

src/tests/stats.vtc

@@ -226,6 +226,6 @@ client c1 {
 
 logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
 	expect 0 * Begin req
-	expect * = VCL_Error {^vmod selector failure: n\.add\(\) may only be called in vcl_init$}
+	expect * = VCL_Error {^vmod selector failure: n\.create_stats\(\) may only be called in vcl_init$}
 	expect * = End
 } -run

src/vmod_selector.c

@@ -744,7 +744,7 @@ vmod_set_create_stats(VRT_CTX, struct vmod_selector_set *set,
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(set, VMOD_SELECTOR_SET_MAGIC);
 	if ((ctx->method & VCL_MET_INIT) == 0) {
-		VFAIL(ctx, "%s.add() may only be called in vcl_init",
+		VFAIL(ctx, "%s.create_stats() may only be called in vcl_init",
 		      set->vcl_name);
 		return;
 	}