docs: fix typo on PhoneAuthActivity.kt (firebase#974)

Author: thatfiredev

auth/app/src/main/java/com/google/firebase/quickstart/auth/kotlin/PhoneAuthActivity.kt

@@ -116,7 +116,7 @@ class PhoneAuthActivity : AppCompatActivity(), View.OnClickListener {
                 // by combining the code with a verification ID.
                 Log.d(TAG, "onCodeSent:$verificationId")
 
-                // Save verification ID and resending token so we can use them latevr
+                // Save verification ID and resending token so we can use them later
                 storedVerificationId = verificationId
                 resendToken = token