docs/help: fix DTD for P2 feature.xml

[chrisrueger]

Replace pipe with comma, since pipe means "or" / choice. But that would mean you can only use one of the elements. Comma means that feature must contain the following elements, which is what we want here

I found an archived site which uses comma: https://archive.eclipse.org/eclipse/downloads/documentation/2.0/html/plugins/org.eclipse.platform.doc.isv/reference/misc/feature_archive.html

[merks]

@akurtakov

I believe this change is an accurate correction according to Elements with Children (sequences) here:

https://www.w3schools.com/xml/xml_dtd_elements.asp

Given it's a doc only change, should we merge it now?

[akurtakov]

Fine by me.

[merks]

@chrisrueger

Thanks for the attention to details!

[chrisrueger]

Thanks @merks
I stumbled over this while working on a testcase the P2Export in bnd and I wanted to add a validation of the feature.xml bnd generates.