Update implementations.md #xquery-coverage-in-exist

[joewiz]

eXist 6.1.0 added support for all of the remaining XQuery 3.1 functions. See:

• eXist 6.1.0 release notes: https://exist-db.org/exist/apps/wiki/blogs/eXist/eXistdb610
• Corresponding update to eXist documentation: https://github.com/eXist-db/documentation/commit/99e589ed8e13a5c19c5e7b003ee8db092f58afd0

What needs to be updated:

• Website: https://coding4humanists.github.io/xquery4humanists/implementations/#xquery-coverage-in-exist
• Source code: https://github.com/coding4humanists/xquery4humanists/blob/release/implementations.md#xquery-coverage-in-exist

[joewiz]

Closed by https://github.com/coding4humanists/xquery4humanists/commit/c2937c7c17cfe6d7c818586b4de3564ecf0b30d5.