Add french translation to 404 page

[ArnolFokam]

Description

This updates the French translation of the 404 page in 404.html. No apparent issues resulting from the changes were noticed and the amp website validations errors were due to my site run locally over HTTP. Check the image below.

Fixes #80

Required checklist:

• My code matches the existing code style of this project.
• One of these:
  • This is a new localization for existing content.
  • This is new content that is localized for all the locales that this project currently supports.
  • I described above how I plan to localize this content immediately after merging these changes.
  • Localization is not applicable to this change.
• I have tested my code and it successfully builds for GitHub Pages and validates as AMP.

[devadvance]

Looks good to me. Really appreciate this contribution! Thanks validating locally and noting the unrelated errors! 😄