clarify PEP reference in "future" page

[brainwane]

Check source/future.rst and ensure that the PEP link for 518 makes sense.

When I was looking at the links more closely, I noticed that in
source/future.rst, on line 32, you've changed the link for PEP 426 to
point to PEP 518, but PEP 518 is less explicitly about a metabuild
system and more foundational (specifying minimum build system
requirements). Could you clarify this in the link wording?

Originally posted by @lgh2 in #24 (comment)