Fix(dockerfile): typo in downloading credhub CLI

[ctlong]

What is this change about?

The url var assignment was missing a semi-colon to delimit the command.

Please provide contextual information.

• https://github.com/cloudfoundry/cf-deployment-concourse-tasks/pull/162
• https://github.com/cloudfoundry/cf-deployment-concourse-tasks/pull/163

Please check all that apply for this PR:

• [ ] introduces a new task
• [ ] changes an existing task
• [x] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

What is the level of urgency for publishing this change?

• [x] Urgent - unblocks current or future work
• [ ] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

None