Rename README.MD to README.md

gui/README.MD → gui/README.md

@@ -23,4 +23,4 @@ This subfolder provides the GUI mode of `pdf2zh`.
   <img src="./img/after.png" width="650" alt="Translated">
 
 ## Maintainance
-GUI maintained by [Rongxin](https://github.com/reycn)
+GUI maintained by [Rongxin](https://github.com/reycn)