Fix broken link in gsod 2021 writeup

Author: natoscott

docs/gsod/2021/casestudy.html

@@ -76,7 +76,7 @@ <h3>About Performance Co-Pilot</h3>
 <h3>About Task-based Guides</h3>
 <p>
 In our
-<a href='https://pcp.io/gsod/2020/proposal.html'>previous</a>
+<a href='https://pcp.io/gsod/2020/ideas.html#readthedocs-pcp'>previous</a>
 Google Season of Docs project, we converted
 two freely-available books (the "Users and Administrators Guide"
 and the "Programmers Guide"), the REST API documentation, and

gsod/2021/casestudy.haml

@@ -40,7 +40,7 @@
           %h3 About Task-based Guides
           %p
             In our
-            %a{:href => 'https://pcp.io/gsod/2020/proposal.html'} previous
+            %a{:href => 'https://pcp.io/gsod/2020/ideas.html#readthedocs-pcp'} previous
             Google Season of Docs project, we converted
             two freely-available books (the "Users and Administrators Guide"
             and the "Programmers Guide"), the REST API documentation, and