fix: correct SSH URL format for private repos

[sienks]

Description

Fixed an issue with SSH protocol URL formatting for private repositories. The code now properly handles SSH URLs when cloning private repositories.

Related Issue(s)

Does not fix an open issue.

Motivation and Context

When trying to load some custom zsh plugins I created from a private repo, zinit was unable to clone from the private repo.

Usage examples

# Using SSH protocol with private repository (existing functionality now works)
zinit ice proto"ssh"
zinit load "username/private-repo"

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• Bug fix (non-breaking change which fixes an issue)
• Documentation change
• New feature (non-breaking change which adds functionality)

Checklist:

• All new and existing tests passed.
• I have added tests to cover my changes.
• I have updated the documentation accordingly.

[sienks]

When I checked, the main branch already has some failing tests. My changes do not introduce any new test failures and shouldn't, as they only modify the URL format.

[pschmitt]

all good, this is going in. Thanks!