Update link to Coursera certificates, and whitelist 403 status codes in htmlproofer

Author: julienrf

.github/workflows/build.yml

@@ -25,7 +25,7 @@ jobs:
           # # Checking for docs.scala-lang/blob/main leads to a chicken and egg problem because of the edit links of new pages.
           bundle exec htmlproofer ./_site/\
              --only-4xx\
-             --http-status-ignore "400,401,429"\
+             --http-status-ignore "400,401,403,429"\
              --empty-alt-ignore\
              --allow-hash-href\
              --url-ignore '/https://github.com/scala/docs.scala-lang/blob/main/.*/,/www.oracle.com/'

learn.md

@@ -19,7 +19,7 @@ four courses and a capstone project.
 All the courses are available for free. Optionally, a subscription gives
 you access to a certificate of completion that attests your accomplishments.
 Learn more about
-[Coursera certificates](https://learner.coursera.help/hc/en-us/articles/209819053-Get-a-Course-Certificate) or
+[Coursera certificates](https://learners.coursera.help/hc/en-us/articles/209819053-Get-a-Course-Certificate) or
 [edX certificates](https://support.edx.org/hc/en-us/categories/115002269627-Certificates).
 
 ### Effective Programming in Scala