docs: Fix a few typos

[timgates42]

There are small typos in:

• braintree/exceptions/server_error.py
• braintree/subscription.py
• braintree/transaction.py
• braintree/us_bank_account_verification_search.py

Fixes:

• Should read submitted rather than submtted.
• Should read statuses rather than statusues.
• Should read multiple rather than mutiple.
• Should read fields rather than fieds.
• Should read existing rather than exisiting.
• Should read contact rather than contant.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[braintreeps]

These fixes were released in 4.13.0. Thanks again, @timgates42!