tiny grammar change (#1457)

Thanks

Author: marktinsley

docs/docs/api/promise.onpossiblyunhandledrejection.md

@@ -14,7 +14,7 @@ Promise.onPossiblyUnhandledRejection(function(any error, Promise promise) handle
 ```
 
 
-*Note: this hook is specific to the bluebird instance its called on, application developers should use [global rejection events](/docs/api/error-management-configuration.html#global-rejection-events)*
+*Note: this hook is specific to the bluebird instance it's called on, application developers should use [global rejection events](/docs/api/error-management-configuration.html#global-rejection-events)*
 
 Add `handler` as the handler to call when there is a possibly unhandled rejection. The default handler logs the error stack to stderr or `console.error` in browsers.
 
@@ -39,4 +39,4 @@ Passing no value or a non-function will have the effect of removing any kind of
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