typo in protobuf readme (#452)

Author: ened

packages/reactive_ble_mobile/protos/README.md

@@ -15,7 +15,7 @@ brew install swift-protobuf
 3. If you don't have Dart SDK on your computer please install it.
 
 ```sh
-brew isntall dart
+brew install dart
 ```
 
 4. Run `pub global activate protoc_plugin`