Update sample app links in README.md

[kemonprogrammer]

Changes

• The links still contained the master branch, which is outdated and caused github to link to the default branch (main). Even though it's correct, it still showed the following warning message:

• The links contained "blob" which refers to files, but they should refer to directories (read on stackoverflow).

• The link to "api-starter" was broken, as discussed by others in this issue: Learn module still refering to the deleted api files #38 , so I updated it to link to the right directory in the mslearn-staticwebapp-api repository.

Additional Notes

• The "api-starter"-link now links to another repository which could be confusing

[saragibby]

Thanks for making these updates!