Fix some typos.

* doc/guix.texi (Sound Services, DNS Services): Fix typos.
* gnu/packages/cpp.scm (json-modern-cxx)[description]: Likewise.
* gnu/packages/emacs.scm (emacs-fancy-narrow)[synopsis, description]:
Likewise. Edit & mark up while we're here.
* gnu/packages/mail.scm (alot)[synopsis, description]: Ditto.