exceptions: fixed typo (#290) [thanks to Maksim Korotkov]

It is implied that command_s should be a string, but now it is a tuple due to a typo.
Signed-off-by: Maksim Korotkov <m.korotkov@postgrespro.ru>

Author: d.kovalenko

src/exceptions.py

@@ -22,7 +22,7 @@ def __str__(self):
             msg.append(self.message)
 
         if self.command:
-            command_s = ' '.join(self.command) if isinstance(self.command, list) else self.command,
+            command_s = ' '.join(self.command) if isinstance(self.command, list) else self.command
             msg.append(u'Command: {}'.format(command_s))
 
         if self.exit_code: