Update metrics.md (#5930)

docs/tendermint-core/metrics.md

@@ -9,7 +9,7 @@ be consumed by Prometheus collector(s).
 
 This functionality is disabled by default.
 
-To enable the Prometheus metrics, set `instrumentation.prometheus=true` if your
+To enable the Prometheus metrics, set `instrumentation.prometheus=true` in your
 config file. Metrics will be served under `/metrics` on 26660 port by default.
 Listen address can be changed in the config file (see
 `instrumentation.prometheus\_listen\_addr`).