fix: missing string ending in python snippet in serving.md

[andreasoprani]

What this PR does / why we need it:

Docs fix: a Python snippet in serving.md had syntax errors.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[seldondev]

Hi @andreasoprani. Thanks for your PR.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[andreasoprani]

/assign @ryandawsonuk

[ukclivecox]

@andreasoprani Thanks!