Updates readme with minor fixes I got stuck on (#330)

CONTRIBUTING.md

@@ -17,6 +17,7 @@ Pre-requisites:
 - Node.js LTS (18.x)
 - [Git LFS](https://git-lfs.github.com/) easiest to install via package manager (e.g. `brew install git-lfs` on macOS)
 - [antlr4-tools](https://github.com/antlr/antlr4-tools) easiest to install with `pip install antlr4-tools` (python3 required)
+- Java (e.g. `brew install java` on macOS)
 
 In the root folder of the project run:
 
@@ -25,7 +26,7 @@ In the root folder of the project run:
 
 From here you can start the `react-codemirror-playground` with:
 
-`npm run dev:codemirror`
+`npm run dev-codemirror`
 
 To run the VS Code extension, choose `VSCode Playground` in the `Run & Debug` menu in VS Code.