Fix formatting in ConnectionFailed error message
Fix formatting in ConnectionFailed error message

Add space at the end of the second line so the complete message separates the word `not` and `working`

message before
```
Stripe::APIConnectionError: Unexpected error communicating when trying to connect to Stripe. You may be seeing this message because your DNS is notworking.  To check, try running `host stripe.com` from the command line.
```
message after
```
Stripe::APIConnectionError: Unexpected error communicating when trying to connect to Stripe. You may be seeing this message because your DNS is not working.  To check, try running `host stripe.com` from the command line.
```
stripe/stripe-ruby