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>

plugins/code-coverage-backend/README.md

@@ -4,7 +4,10 @@ This is the backend part of the `code-coverage` plugin. It takes care of process
 
 ## Installation
 
-`yarn workspace backend add @backstage/plugin-code-coverage-backend`
+```sh
+# From your Backstage root directory
+yarn add --cwd packages/backend @backstage/plugin-code-coverage-backend
+```
 
 First create a `codecoverage.ts` file here: `packages/backend/src/plugins`. Now add the following as its content: