fix: use correct schemaLocation for urlset (v4 compat)

[redjam13]

Description

@ThePixelDeveloper re PR https://github.com/ThePixelDeveloper/Sitemap/pull/69 - it seems this has been tagged in 4.5.6 but it looks like it should have been tagged as 5.1.4?

The change in this PR would be the change needed if we wanted to patch 4.*

I think this may be breaking for anyone on ^4 - is it possible to delete the 4.5.6 tag / make a new one with the changes in this PR?

[rogamoore]

@ThePixelDeveloper Would it possible to tag a new version 5.1.4 as suggested by @redjam13 ?

[JoshBour]

I agree, as it is you have to use dev-master or 4.5.6 to make it work.