Normalize a few instances of "T-shirt" to "t-shirt"

The latter is overwhelmingly more common.

lib/convert-worker.js

@@ -423,6 +423,9 @@ function getBodyXML(chapter, book, contentEl) {
   // Clich(e|é) is spelled both ways. Let's standardize on including the accent.
   xml = xml.replace(/cliche/g, "cliché");
 
+  // T-shirt is usually spelled lowercase ("t-shirt"). Normalize the remaining instances.
+  xml = xml.replace(/T-shirt/g, "t-shirt");
+
   // "gray" is the majority spelling, except for "greyhound"
   xml = xml.replace(/(G|g)rey(?!hound)/g, "$1ray");