doc: fix body length in SMP UART transport

[smnmsr]

In the description of the partial-final frame of the SMP UART transport specification was a typo in the maximum allowed raw body size. The size must not be larger than MTU-5 because there are 5 bytes occupied by total length (2), frame termination (1) and crc (2).

[github-actions]

Hello @smnmsr, and thank you very much for your first pull request to the Zephyr project!
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[de-nordic]

@smnmsr Fix commit message to make compliance happy.

[nordicjm]

@smnmsr please fix compliance issue

[mmahadevan108]

@smnmsr can you help fix the CI compliance check failures.

[kartben]

@smnmsr for the record: `git commit --amend --author="First Last your@email.com" :)

@mmahadevan108 should be good to go now!

[github-actions]

Hi @smnmsr!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁