Fix suffix of milliseconds formatter

src/format.cxx

@@ -64,7 +64,7 @@ namespace logging {
         fmtstring.replace(pos, 1, "%%");
         pos += 2;
       }
-      fmtstring.replace(fmtstring.find("%%_ms"), 4, "%.03d");
+      fmtstring.replace(fmtstring.find("%%_ms"), 5, "%.03d");
       return [=](logpacket const & lp) {
         return fmt_time_with_milis(lp.time, fmtstring);
       };