Tighten a cross-reference in documentation

Tighten a cross-reference in documentation

* doc/lispref/internals.texi (Writing Emacs Primitives): Switch to
  a simple parenthetical cross-reference, following up to my
  commit 9a53b6d426 of 2018-06-24.

See discussion:

  https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00826.html
  From: Eli Zaretskii
  Subject: Re: [Emacs-diffs] \
           emacs-26 9a53b6d: Say how to override a primitive interactive spec
  To: Karl Fogel
  CC: Stefan Monnier, Emacs Devel
  Date: Mon, 25 Jun 2018 17:41:53 +0300
  Message-Id: <83r2kvrkr2.fsf@gnu.org>