Fixed gcc warning

include/spdlog/details/fmt_helper.h

@@ -115,7 +115,7 @@ inline void pad6(size_t n, fmt::memory_buffer &dest)
         dest.push_back('0');
         dest.push_back('0');
     }
-    else if (n >= 0)
+    else // 0-9
     {
         dest.push_back('0');
         dest.push_back('0');