Update gitlab docs deploy_tokens index.md to use https as git clone url for better security

What does this MR do?

Updates the gitlab docs page about deploy_tokens to use https for the git clone url for better security. Many people will copy this command so it will be a win in the long run.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry => Any docs-only changes should not have a changelog entry.
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

The only risk this change involves is that git clone will not work if the server doesn't support https. The user will get a clear error if this happens so no tests etc. are required.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team