Merge pull request #18558 from oliviertassinari/mui-material-ui

fix: use Material UI vs. MUI where relevant

docs/plugins/plugin-development.md

@@ -16,7 +16,7 @@ browser APIs or by depending on external modules to do the work.
 - Consider writing plugins in `TypeScript`.
 - Plan the directory structure of your plugin so that it becomes easy to manage.
 - Prefer using the [Backstage components](https://backstage.io/storybook),
-  otherwise go with [Material-UI](https://material-ui.com/).
+  otherwise go with [Material UI](https://material-ui.com/).
 - Check out the shared Backstage APIs before building a new one.
 
 ## Plugin concepts / API

docs/plugins/structure-of-a-plugin.md

@@ -93,7 +93,7 @@ next to them you can split up the UI in as many components as you feel like.
 
 We have the `ExampleComponent` to show an example Backstage page component. The
 `ExampleFetchComponent` showcases the common task of making an async request to
-a public API and plot the response data in a table using Material-UI components.
+a public API and plot the response data in a table using Material UI components.
 
 You may tweak these components, rename them and/or replace them completely.