make_flake_definition: replace the first : with a /

[grahamc]

Converts git+ssh://[email protected]:DeterminateSystems/xxx.git?ref=grahamc-patch-1 to git+ssh://[email protected]/DeterminateSystems/xxx.git?ref=grahamc-patch-1

Description

Checklist

• Checked the rust with cargo build, cargo test, cargo fmt, and cargo clippy in the jobset-generator directory
• Verifed the example configuration still parses with terraform init && terraform validate in the terraform directory
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)