Fix an issue in IDM_EDIT_CLEARREADONLY

Using ^= for the readonly attribute will cause it to always change the bit.  So if CLEARREADONLY is called on a file that isn't read only, the file will become read only.

Fix #5768, close #5774

PowerEditor/src/NppCommands.cpp

@@ -1568,7 +1568,7 @@ void Notepad_plus::command(int id)
 			Buffer * buf = _pEditView->getCurrentBuffer();
 
 			DWORD dwFileAttribs = ::GetFileAttributes(buf->getFullPathName());
-			dwFileAttribs ^= FILE_ATTRIBUTE_READONLY;
+			dwFileAttribs &= ~FILE_ATTRIBUTE_READONLY;
 
 			::SetFileAttributes(buf->getFullPathName(), dwFileAttribs);
 			buf->setFileReadOnly(false);