minimal-printf: README correction

[rajkan01]

Summary of changes

• README description is updated to refer target.printf_lib for minimal-printf library selection instead of target.printf.

Impact of changes

With these changes, user gets correct information about minimal-printf library selection.

Migration actions required

None.

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@evedon

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[ciarmcom]

@rajkan01, thank you for your changes.
@evedon @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[kjbracey]

CI started

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

I've fixed the version: Set to 6.0.0-alpha-2