Fix invalid XML namespace

[spideyfusion]

Although it's possible to view the web page contents of the referenced namespace, the only valid name for the namespace is http://www.w3.org/1999/xhtml.

The XML namespace specification is clear when a namespace reference is considered identical:

[Definition: The two URIs are treated as strings, and they are identical if and only if the strings are identical, that is, if they are the same sequence of characters. ] The comparison is case-sensitive, and no %-escaping is done or undone.

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The error is evident from the GSC (Google Search Console) when it analyzes a sitemap generated by the library:

Your Sitemap or Sitemap index file doesn't properly declare the namespace. Expected: http://www.w3.org/1999/xhtml Found: https://www.w3.org/1999/xhtml

[coveralls]

Coverage remained the same at 96.925% when pulling e13121b on spideyfusion:fix-invalid-namespace into ddc67fb on gpslab:2.x.

[peter-gribanov]

Good job. Thanks 👍