fix encoding_to_xmlm_encoding typo

Author: zazedd

src/xmlt.ml

@@ -10,5 +10,5 @@ type encoding = [ `UTF_8 | `UTF_16 | `UTF_16BE | `UTF_16LE ]
 let encoding_to_xmlm_encoding : encoding -> Xmlm.encoding = function
   | `UTF_8 -> `UTF_8
   | `UTF_16 -> `UTF_16
-  | `UTF_16BE -> `UTF_16LE
-  | `UTF_16LE -> `UTF_16BE
+  | `UTF_16LE -> `UTF_16LE
+  | `UTF_16BE -> `UTF_16BE