add missing PR href

Author: jquast

README.rst

@@ -218,6 +218,7 @@ http://www.cl.cam.ac.uk/~mgk25/ucs/wcwidth.c::
 .. _`PR #4`: https://github.com/jquast/wcwidth/pull/4
 .. _`PR #5`: https://github.com/jquast/wcwidth/pull/5
 .. _`PR #11`: https://github.com/jquast/wcwidth/pull/11
+.. _`PR #18`: https://github.com/jquast/wcwidth/pull/18
 .. _`jquast/blessed`: https://github.com/jquast/blessed
 .. _`jonathanslenders/python-prompt-toolkit`: https://github.com/jonathanslenders/python-prompt-toolkit
 .. _`wcwidth(3)`:  http://man7.org/linux/man-pages/man3/wcwidth.3.html