fix exisits typo (#11989)

Author: paulb777

FirebaseAuth/Tests/Unit/FIRUserTests.m

@@ -2997,7 +2997,7 @@ - (void)testlinkPhoneAuthCredentialFailure {
 
 /** @fn testlinkPhoneCredentialAlreadyExistsError
     @brief Tests the flow of @c linkWithCredential:completion:
-        call using a phoneAuthCredential and a credential already exisits error. In this case we
+        call using a phoneAuthCredential and a credential already exists error. In this case we
         should get a FIRAuthCredential in the error object.
  */
 - (void)testlinkPhoneCredentialAlreadyExistsError {

FirebaseMessaging/CHANGELOG.md

@@ -1,5 +1,5 @@
 # 10.12.0
-- [changed] Removing fiam scoped tokens set by old FIAM SDK(s) from keychain if exisits (b/284207019).
+- [changed] Removing fiam scoped tokens set by old FIAM SDK(s) from keychain if exists (b/284207019).
 
 # 10.6.0
 - [fixed] Configure flow validates existence of an APNS token before fetching an FCM token (#10742). This also addresses the scenario 1 mentioned in the comment - https://github.com/firebase/firebase-ios-sdk/issues/10679#issuecomment-1402776795