chore: fix missing closing section tag in README

[pxxad]

This PR fixes a missing closing tag in the README for the geometric logpmf distribution.
Ref #5714

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[kgryte]

LGTM after clean-up

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/geometric/logpmf	$\color{green}289/289$ $\color{green}+100.00%$	$\color{green}24/24$ $\color{green}+100.00%$	$\color{green}4/4$ $\color{green}+100.00%$	$\color{green}289/289$ $\color{green}+100.00%$

The above coverage report was generated for the changes in this PR.