Update README.md

README.md

@@ -197,7 +197,7 @@ flexible, while being designed for consistency and RFC-compliance. Adding custom
 health checks is very easy. All you need to do is to call an `addCheck()` method
 with the name of the component and metrics that the check is related to, as well
 as pass a health check executor function that will be executed to retrieve the
-health values. Executor function must be a function that returns a fresh promise
+health values. The executor function must be a function that returns a fresh promise
 at every execution (usually: it is an ES2018 async function).
 
 The return JSON object MUST have `status` field that is either 'pass', 'warn'