fix typo in docstring

<div id="comment:0"></div>



Component: **documentation**

Author: **Martin Rubey**

Branch/Commit: **[`6cb4043`](https://github.com/sagemath/sagetrac-mirror/commit/6cb40430327f5e7ac5addd78841119abdedcb916)**

Reviewer: **Frédéric Chapoton**

_Issue created by migration from https://trac.sagemath.org/ticket/34571_

