fix flutter installation command: pub get -> pub add

Author: imtushardave

templates/dart/README.md.twig

@@ -35,7 +35,7 @@ dependencies:
 You can install packages from the command line:
 
 ```bash
-pub get {{ language.params.packageName }}
+dart pub add {{ language.params.packageName }}
 ```
 
 {% if sdk.gettingStarted %}

templates/flutter/README.md.twig

@@ -35,7 +35,7 @@ dependencies:
 You can install packages from the command line:
 
 ```bash
-pub get {{ language.params.packageName }}
+flutter pub add {{ language.params.packageName }}
 ```
 
 {% if sdk.gettingStarted %}