fix: Typo in 714c119 using f-string (follow-up of follow-up of #1938)

[rapsealk]

This PR is a follow-up to 714c119 to supply missing "f-string".

backend.ai/src/ai/backend/client/session.py

Lines 68 to 74 in 9e56e54

	)
	if server_version < MIN_API_VERSION:
	warnings.warn(
	"The server is too old ({server_version}) and does not meet the minimum API version"
	f" requirement: v{MIN_API_VERSION[0]}.{MIN_API_VERSION[1]}\nPlease upgrade"
	" the server or downgrade/reinstall the client SDK with the same"
	" major.minor release of the server.",

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[achimnol]

Thanks!