Fixed docstring in synchronous serial client connect() method

I think there was a copy/paste error in the docstring. It said tcp but I think it is supposed to be serial.

Author: idahogray

pymodbus/client/sync.py

@@ -310,7 +310,7 @@ def __implementation(method):
         raise ParameterException("Invalid framer method requested")
 
     def connect(self):
-        ''' Connect to the modbus tcp server
+        ''' Connect to the modbus serial server
 
         :returns: True if connection succeeded, False otherwise
         '''