Update docs to always use yarn add --cwd for app & backend

Some commands currently error, and others are simply formatted inconsistently.
This format seems to work and be the most popular/common,
so I applied it to all similar occurrences.

Signed-off-by: Jan Van Bruggen <JanVB@verily.com>

docs/plugins/backend-plugin.md

@@ -66,7 +66,7 @@ To actually attach and run the plugin router, you will make some modifications
 to your backend.
 
 ```bash
-# From the Backstage root directory
+# From your Backstage root directory
 yarn add --cwd packages/backend @internal/plugin-carmen-backend@^0.1.1 # Change this to match the plugin's package.json
 ```