Replace inexistant lightmagenta CSS colour with hotpink

This improves compatibility with clients respecting the CSS
specification.

Thanks PulkoMandy, who found it using libcss in Renga!

src/bridge/colors.cpp

@@ -24,7 +24,7 @@ static const char* irc_colors_to_css[IRC_NUM_COLORS] = {
   "cyan",
   "lightcyan",
   "lightblue",
-  "lightmagenta",
+  "hotpink",
   "gray",
   "white",
 };