Say "internal link" instead of "internal URL"
Created by: seancolsen
@joannechester pointed out that I've been using "URL" to refer to things that are not URLs! Wow! I never knew!
Per https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#URI_references it seems that something like /foo/bar.md
isn't technically a "URL". So to be more accurate with our language, this PR changes the meta docs to refer to internal "links" instead.
Note that I intentionally preserved the anchor #internal-url-standards
to avoid broken references.
What do you think @joannechester? Is this the sort of change you had in mind?