Update the xmlns="http://www.w3.org/1999/xhtml/" in all mapml files

[AliyanH]

After #887 (where map-extent is established as a custom element) we discovered that a few remote files were not working properly this is because the xmlns was set incorrectly and had a trailing back slash.

We need to update mapml files across all repos to remove the trailing back slash, to go from:

xmlns="http://www.w3.org/1999/xhtml/"

to

xmlns="http://www.w3.org/1999/xhtml"

[yhy0217]

Resolved in #887