Fix javadoc link.

[hawjo06]

The link was generated as plaintext. The change will result in the link
being rendered as a hyperlink.

[cpovirk]

javac9 -Xdoclint rejects this version :(

It sounds like what we have is supposed to work (or at least ImmutableTable.Builder#Builder, perhaps?), but javadoc8 doesn't understand it. Oddly, it looks like javadoc9 doesn't understand it, either.... Maybe I'm doing something wrong.

[cpovirk]

Hmm, no, I was doing something wrong, though I'm not 100% sure what it was :) I'll see if I can at least get it to work with javadoc9 now.

[cpovirk]

OK, the code that we currently have in Github works under javadoc9. We can't switch to the new code without breaking our internal builds (which use java9 with -Xdoclint) -- and, it appears, breaking javadoc9. The easiest solution here is probably to wait for our switch to using Java 9 for our open-source builds.

[hawjo06]

@cpovirk Thanks for looking into this, and the detailed responses. I will close this PR.