update the protoc version to 3.12.3 in readme.md
@@ -4,6 +4,6 @@ re-compile them when building Prometheus.
If however you have modified the defs and do need to re-compile, run
`make proto` from the parent dir.
-In order for the script to run, you'll need `protoc` (version 3.5.1) in your
+In order for the script to run, you'll need `protoc` (version 3.12.3) in your
PATH.
