Fix documentation link in README

[stomar]

Provide an unversioned link that always
points to the docs for the latest Ruby.

[I missed this in my last PR.]

Alternatives to https://ruby-doc.org/stdlib/libdoc/fileutils/rdoc/FileUtils.html (latest stable release) would be:

• https://docs.ruby-lang.org/en/trunk/FileUtils.html (Ruby trunk docs, daily build)
• http://www.rubydoc.info/gems/fileutils/FileUtils (latest gem release docs)

(ruby-doc.org also has a stdlib-trunk branch, but I'm not sure whether this is kept up-to-date.)

If you prefer one of those, I can amend the PR.

[amatsuda]

@stomar Thank you!
Linking to ruby-doc.org seems good since that's the one linked from https://www.ruby-lang.org/en/documentation/.

[stomar]

Good catch... docs.ruby-lang.org is not mentioned there; need to fix that.