Fix links to Why Aren't PipelineResources in Beta?

Backport #4572

Links to the "Why Aren't PipelineResources in Beta?" section in the docs
should have `aren-t` in the fragment instead of `arent`. This can be
confirmed by clicking the link icon beside the heading and checking the
browser address bar.

docs/migrating-v1alpha1-to-v1beta1.md

@@ -80,7 +80,7 @@ Since then, **`PipelineResources` have been deprecated**. We encourage users to
 features instead of `PipelineResources`. Read more about the deprecation in [TEP-0074](https://github.com/tektoncd/community/blob/main/teps/0074-deprecate-pipelineresources.md).
 
 _More on the reasoning and what's left to do in
-[Why aren't PipelineResources in Beta?](resources.md#why-arent-pipelineresources-in-beta)._
+[Why aren't PipelineResources in Beta?](resources.md#why-aren-t-pipelineresources-in-beta)._
 
 To ease migration away from `PipelineResources`
 [some types have an equivalent `Task` in the Catalog](#replacing-pipelineresources-with-tasks).

docs/resources.md

@@ -33,7 +33,7 @@ For example:
 > which lists each PipelineResource type and a suggested option for replacing it.
 >
 > For more information on why PipelineResources are remaining alpha [see the description
-> of their problems, along with next steps, below](#why-arent-pipelineresources-in-beta).
+> of their problems, along with next steps, below](#why-aren-t-pipelineresources-in-beta).
 
 --------------------------------------------------------------------------------
 
@@ -52,7 +52,7 @@ For example:
     -   [Storage Resource](#storage-resource)
         -   [GCS Storage Resource](#gcs-storage-resource)
     -   [Cloud Event Resource](#cloud-event-resource)
--   [Why Aren't PipelineResources in Beta?](#why-arent-pipelineresources-in-beta)
+-   [Why Aren't PipelineResources in Beta?](#why-aren-t-pipelineresources-in-beta)
 
 ## Syntax