correct a few typoes

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

including several occurrences of "empy"

Component: **documentation**

Author: **Frédéric Chapoton**

Branch/Commit: **[`2b93625`](https://github.com/sagemath/sagetrac-mirror/commit/2b93625c16069d88858038088702ef1f70921bf8)**

Reviewer: **Nathann Cohen**

_Issue created by migration from https://trac.sagemath.org/ticket/19600_

