minor typo fix in auth-flow (react-navigation#782)

The sentence "This can usually be done by checking if we have a token in `AsyncStorage` and validating if token" is a bit confusing. Specifically "validating if token". I believe it's implied that we check if the token exists and validate it.

Author: nodes777

versioned_docs/version-5.x/auth-flow.md

@@ -88,7 +88,7 @@ return (
 );
 ```
 
-In the above snippet, `isLoading` means that we're still checking if we have a token. This can usually be done by checking if we have a token in `AsyncStorage` and validating if token. After we get the token and if it's valid, we need to set the `userToken`. We also have another state called `isSignout` to have a different animation on sign out.
+In the above snippet, `isLoading` means that we're still checking if we have a token. This can usually be done by checking if we have a token in `AsyncStorage` and validating the token. After we get the token and if it's valid, we need to set the `userToken`. We also have another state called `isSignout` to have a different animation on sign out.
 
 The main thing to notice is that we're conditionally defining screens based on these state variables: