Update URL on sitemap

[ljwalker]

In sitemap.php:

The link to "Symbiota Documentation" currently links to a defunct page on symbiota.org. This URL should point to https://biokic.github.io/symbiota-docs/col_obs/ and the wording should be updated.

Old text:

Visit the Symbiota Documentation for more information on specimen processing capabilities.

Suggested text:

Visit the Collector/Observer Guide to learn more about managing observational records in Symbiota portals.