Rename the FR Appel à Contributions blog post

[charlottejmc]

I am preparing to change the title of the blog post from « Appel à Contributions » to « Appel à Propositions ».

Across our site and social media, we've used the term 'contribution' to refer to any work by authors, translators, reviewers, and editors. The term 'proposition' (proposal) is better suited to refer specifically to the proposal of a new lesson/translation by an author/translator.