fix(cli): show one replica URL in usage

[corylanou]

Description

Updates the replicate command usage to show a single REPLICA_URL, matching the supported command-line behavior. No parsing or runtime behavior changes.

Motivation and Context

The help output advertised optional additional replica URLs even though multiple replicas on one database are rejected at runtime. The rendered usage now shows DB_PATH REPLICA_URL without the repeatable argument.

Fixes #1350

How Has This Been Tested?

• go run ./cmd/litestream replicate -h
• go test -race ./...
• pre-commit run --all-files

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[github-actions]

PR Build Metrics

✅ All clear — no issues detected

Check	Status	Summary
Binary size	✅	36.95 MB (0.0 KB / 0.00%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.12 (latest)
Module graph	✅	1240 edges (0)

Binary Size

	Size	Change
Base (c96c0f4)	36.95 MB
PR (bc44993)	36.95 MB	0.0 KB (0.00%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 0 vulnerabilities in packages you import and 1
vulnerability in modules you require, but your code doesn't appear to call these
vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	43s
Go version	go1.25.12
Commit	bc44993

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_{🤖 Updated on each push.}