Address minor nits in new frontend system docs

Fix some small typos, correct some casing, and add a link to
an area that I struggled to understand without guidance.

Signed-off-by: Mitchell Hentges <mhentges@spotify.com>

docs/frontend-system/building-plugins/02-testing.md

@@ -70,7 +70,7 @@ describe('Entity details component', () => {
 
     await renderInTestApp(
       <TestApiProvider apis={[[catalogApiRef, catalogApiMock]]}>
-        <EntityDetails entitRef={entityRef} />
+        <EntityDetails entityRef={entityRef} />
       </TestApiProvider>,
     );