Update prompb/README.md

Co-authored-by: Julien Pivotto <roidelapluie@o11y.eu>
Signed-off-by: Paschalis Tsilias <tpaschalis@users.noreply.github.com>

prompb/README.md

@@ -4,7 +4,7 @@ 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 3.15.8) in
+In order for the [script][../scripts/genproto.sh] to run, you'll need `protoc` (version 3.15.8) in
 your PATH.
 
 [script]: ../scripts/genproto.sh