docs: update CLA links to point to Docusign form.

The openedx.org/cla link is a redirection to Docusign.  If the form has
to change in the future, we can update the redirect at openedx.org.

There are other changes to make to CONTRIBUTING.rst, but those will have
to wait.

CONTRIBUTING.rst

@@ -100,11 +100,11 @@ Step 1: Sign a Contribution Agreement
 =====================================
 
 Before edX can accept any code contributions from you, you'll need to sign the
-`individual contributor agreement`_. This confirms that you have the authority
+`Individual Contributor Agreement`_. This confirms that you have the authority
 to contribute the code in the pull request and ensures that edX can re-license
 it.
 
-.. _individual contributor agreement: https://openedx.org/cla
+.. _Individual Contributor Agreement: https://openedx.org/cla
 
 If you will be contributing code on behalf of your employer or another
 institution you are affiliated with, please reach out by email to [email protected]