Hint at mem(4) in error message

Index: util/cbmem/cbmem.c
--- util/cbmem/cbmem.c.orig
+++ util/cbmem/cbmem.c
@@ -1811,7 +1811,7 @@ int main(int argc, char** argv)
 
 	mem_fd = open("/dev/mem", timestamp_id ? O_RDWR : O_RDONLY, 0);
 	if (mem_fd < 0) {
-		fprintf(stderr, "Failed to gain memory access: %s\n",
+		fprintf(stderr, "Failed to read /dev/mem, see mem(4): %s\n",
 			strerror(errno));
 		return 1;
 	}
