Add XML as an initialism.

Fixes issue #1.
diff --git a/lint.go b/lint.go
index de2a2a1..51db210 100644
--- a/lint.go
+++ b/lint.go
@@ -339,6 +339,7 @@
 	"IP":    true,
 	"JSON":  true,
 	"URL":   true,
+	"XML":   true,
 }
 
 // lintTypeDoc examines the doc comment on a type.