docs(CONTRIBUTING): update links (#932)

* docs(CONTRIBUTING): update links

... the old egghead.io link redirects to https://app.egghead.io/playlists/how-to-contribute-to-an-open-source-project-on-github
which displays "We've got a new version of egghead in BETA. Click here
to try it!"

remove obsolete all-contributors link (was removed in 29aad25)

* docs: use consistent spelling for TypeScript

... inspired by https://effectivetypescript.com/2021/02/03/pet-peeves/

Co-authored-by: Matan Borenkraout <Matanbobi@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -37,7 +37,7 @@ merge of your pull request!
 - [ ] Documentation added to the
       [docs site](https://github.com/testing-library/testing-library-docs)
 - [ ] Tests
-- [ ] Typescript definitions updated
+- [ ] TypeScript definitions updated
 - [ ] Ready to be merged
       <!-- In your opinion, is this ready to be merged as soon as it's reviewed? -->
 

CONTRIBUTING.md

@@ -35,11 +35,11 @@ sure to include those changes (if they exist) in your commit.
 ### Update Typings
 
 If your PR introduced some changes in the API, you are more than welcome to
-modify the Typescript type definition to reflect those changes. Just modify the
-`/types/index.d.ts` file accordingly. If you have never seen Typescript
+modify the TypeScript type definition to reflect those changes. Just modify the
+`/types/index.d.ts` file accordingly. If you have never seen TypeScript
 definitions before, you can read more about it in its
 [documentation pages](https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html).
-Though this library itself is not written in Typescript we use
+Though this library itself is not written in TypeScript we use
 [dtslint](https://github.com/microsoft/dtslint) to lint our typings.
 
 ## Help needed
@@ -50,6 +50,5 @@ Also, please watch the repo and respond to questions/bug reports/feature
 requests! Thanks!
 
 [egghead]:
-  https://app.egghead.io/series/how-to-contribute-to-an-open-source-project-on-github
-[all-contributors]: https://github.com/all-contributors/all-contributors
+  https://egghead.io/courses/how-to-contribute-to-an-open-source-project-on-github
 [issues]: https://github.com/testing-library/react-testing-library/issues