Fix Readme `pub get` instead of `pub install`. (#486)

Co-authored-by: Ömer Sinan Ağacan <omersa@google.com>
diff --git a/protoc_plugin/README.md b/protoc_plugin/README.md
index 14bc78f..ce98b99 100644
--- a/protoc_plugin/README.md
+++ b/protoc_plugin/README.md
@@ -25,7 +25,7 @@
 *Note:* currently the workflow is POSIX-oriented.
 
 To build standalone `protoc` plugin:
-- run `pub get` to install all dependencies
+- Run `dart pub get` to install all dependencies
 - Now you can use the plugin either by adding the `bin` directory to your `PATH`,
   or passing it directly with `protoc`'s `--plugin` option.