emacs: fix docstring for `notmuch-search-line-faces'.

Examples in documentation for `notmuch-search-line-faces' had an extra
quote, e.g.:

  '(\"unread\" . '(:foreground \"green\"))

Which resulted in values like:

  (\"unread\" quote (:foreground \"green\"))

And tons of "Invalid face reference: quote" errors in the messages
buffer.