Fixed typo: 'lenght' in coap_service_api.h

[DavidLin1577]

Note that the word 'lenght' is wrong,
so that 'lenght' should been replaced with 'length'.

Summary of changes

Impact of changes

Migration actions required

Documentation

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Pull request type

[*] 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

[*] 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

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[ciarmcom]

@DavidLin1577, thank you for your changes.
@ARMmbed/mbed-os-mesh @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
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