Fix link of read-the-docs

[aweinstein]

The Read the Docs is linked to branch 22-change-basename-folder-to-pygsp2. It should point to the branch master.

Note that the link we have is https://pygsp2.readthedocs.io/en/22-change-basename-folder-to-pygsp2/index.html#

It should be:

https://pygsp2.readthedocs.io/en/stable/#

[lcortesg]

Doc is linked to 22-change-basename-folder-to-pygsp2 branch until the pending merge to the master branch is approved, until that, the Read the Docs build for the master branch will keep failing, hence the broken link for the latest release https://pygsp2.readthedocs.io/en/latest/

[cjotade]

Other option is to create a "stable" branch that has the same information in 22-change-basename-folder-to-pygsp2

[aweinstein]

I did a review on #25 As soon as @lcortesg and @cjotade address my comments, we can do the merge and fix this issue.