Fix bug in cancel_order due to wrong concatenation syntax

[Te12944265-AMAHA]

Checklist

• [x] Added tests for changed code.
• [x] Updated documentation for changed code.
• [x] I've added a news fragment of my changes with the name, "{ISSUE_NUM}.{feature|bugfix|doc|removal|misc}""

Related Issue

Fixes #280, fixes #281

Description

Cancel order throws an error as follows:

----> 1 cancel_order = rh.cancel_order(order_id)

File ~/.local/lib/python3.8/site-packages/pyrh/robinhood.py:1474, in Robinhood.cancel_order(self, order_id)
   1472 if isinstance(order_id, str):
   1473     try:
-> 1474         order = self.get(urls.build_orders() + order_id)
   1475     except (requests.exceptions.HTTPError) as err_msg:
   1476         raise ValueError(
   1477             "Failed to get Order for ID: "
   1478             + order_id
   1479             + "\n Error message: "
   1480             + repr(err_msg)
   1481         )

TypeError: unsupported operand type(s) for +: 'URL' and 'str'

Verification

With the new changes, an order can be canceled successfully.