libutil: Fix error message

I encountered this with a misconfigured libutil. I doubt that a
non-lutimes config is viable, because tests were failing.

src/libutil/file-system.cc

@@ -682,7 +682,7 @@ void setWriteTime(
         if (utimes(path.c_str(), times) == -1)
             throw SysError("changing modification time of %s (not a symlink)", path);
     } else {
-        throw Error("Cannot modification time of symlink %s", path);
+        throw Error("Cannot change modification time of symlink %s", path);
     }
 #endif
 #endif