feat(docs): update flutter fn names

Author: jjarvisp

src/pages/[platform]/build-a-backend/auth/concepts/multi-factor-authentication/index.mdx

@@ -872,7 +872,7 @@ await signUp({
 
 <InlineFilter filters={["flutter"]}>
 ```dart
-Future<void> signUpWithPhoneVerification(
+Future<void> signUpWithEmailVerification(
   String username,
   String password,
 ) async {
@@ -932,7 +932,7 @@ await confirmSignUp({
 
 <InlineFilter filters={["flutter"]}>
 ```dart
-Future<void> confirmSignUpPhoneVerification(
+Future<void> confirmSignUpEmailVerification(
   String username,
   String otpCode,
 ) async {