Fix comment for initializer for FIRInAppMessagingAction after deprecation (#3765)

Author: christibbs

Firebase/InAppMessaging/Public/FIRInAppMessagingRendering.h

@@ -135,7 +135,7 @@ NS_SWIFT_NAME(InAppMessagingAction)
 /// Unavailable.
 - (instancetype)init NS_UNAVAILABLE;
 
-/// Deprecated, this class shouldn't be directly instantiated.
+/// This class should only be initialized from a custom in-app message UI component implementation.
 - (instancetype)initWithActionText:(nullable NSString *)actionText actionURL:(NSURL *)actionURL;
 
 @end