Remove broken link to colour overview

Author: thomasp85

vignettes/ggplot2-specs.Rmd

@@ -19,9 +19,7 @@ This vignette summarises the various formats that grid drawing functions take.
 Almost every geom has either colour, fill, or both. Colours and fills can be specified in the following ways:
 
 *   A __name__, e.g., `"red"`. R has `r length(colours())` built-in named
-    colours, which can be listed with `colours()`. The Stowers Institute 
-    provides a nice printable pdf that lists all colours: 
-    <http://research.stowers.org/mcm/efg/R/Color/Chart/>.
+    colours, which can be listed with `colours()`.
 
 *   An __rgb specification__, with a string of the form `"#RRGGBB"` where each of 
     the pairs `RR`, `GG`, `BB` consists of two hexadecimal digits giving a value