Update date format on sitemap page

[yielderdev]

Fixes #8489

What changes did you make?

• Edited the Site Map date 'as of' format for consistency
• Updated the date format from MM/DD/YY to YYYY-MM-DD
• Did not change the 'as of' date itself, just the date formatting.

Why did you make the changes (we will use this info to test)?

• Our website needs to use consistent formatting in our program and process.

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From your project repository, check out a new branch and test the changes.

git checkout -b yielderdev-fix-sitemap-date-8489 gh-pages
git pull https://github.com/yielderdev/HackForLA-website.git fix-sitemap-date-8489