Fix "grey" to "gray" (occurs four times)

Closes #11.

lib/convert.js

@@ -337,6 +337,9 @@ function getBodyXML(chapter, contentEl) {
   // Clich(e|é) is spelled both ways. Let's standardize on including the accent.
   xml = xml.replace(/cliche/g, "cliché");
 
+  // "gray" is the majority spelling, except for "greyhound"
+  xml = xml.replace(/(G|g)rey(?!hound)/g, "$1ray");
+
   // These are consistently missing hyphens.
   xml = xml.replace(/self destruct/g, "self-destruct");
   xml = xml.replace(/life threatening/g, "life-threatening");