docs: split install commands for Reason packages

This allows use to pick which install command to copy instead of both.

Author: nickserv

docs/bs-react-testing-library/intro.mdx

@@ -15,6 +15,9 @@ Bindings for several testing libraries have been ported to [ReasonML][re].
 
 ```bash npm2yarn
 npm install --save-dev bs-dom-testing-library
+```
+
+```bash npm2yarn
 npm install --save-dev bs-react-testing-library
 ```