Update reference to envelope.body

which now should be body_txt as of e067ea2

fixes pazz#1468

alot/commands/globals.py

@@ -869,7 +869,7 @@ async def _set_signature(self, ui):
                     mimetype = helper.guess_mimetype(sigcontent)
                     if mimetype.startswith('text'):
                         sigcontent = helper.try_decode(sigcontent)
-                        self.envelope.body += '\n' + sigcontent
+                        self.envelope.body_txt += '\n' + sigcontent
             else:
                 ui.notify('could not locate signature: %s' % sig,
                           priority='error')