Fix recommended protoc version

Signed-off-by: Paschalis Tsilias <paschalist0@gmail.com>

prompb/README.md

@@ -4,6 +4,8 @@ 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 3.12.3) in your
+In order for the [script][] to run, you'll need `protoc` (version 3.15.8) in
-PATH.
+your PATH.
+
+[script]: ../scripts/genproto.sh