Update specification-reference.md: Removes homepage suggestion under metadata

[jkylekelly]

This pull request updates the specification-reference.md file to revise the metadata section of the gem specification. The change removes references to the gem’s homepage from both the explanatory text and the example metadata.

As a first time reader, I was shocked that homepage was suggested under metadata. But then I realized it did in fact have a dedicated attribute. Suggesting to remove it from the metadata example to reduce confusion.

[deivid-rodriguez]

I think this makes sense, however, there is a metadata attribute too for the homepage and it actually takes priority over the dedicated attribute.

We have some issues about it in rubygems.org, see rubygems/rubygems.org#5544 or rubygems/rubygems.org#4924.

I think this documentation change may actually make those issues more common, so we should probably try fix the linked issues first before changing docs?

[jkylekelly]

@deivid-rodriguez I wasn't aware of the issues mentioned - Agreed that they should probably be fixed first! Surprised to learn the metadata attribute takes priority.