fix some details in graphs

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

found by lgtm

CC:  @dcoudert

Component: **graph theory**

Author: **Frédéric Chapoton**

Branch/Commit: **[`2d79307`](https://github.com/sagemath/sagetrac-mirror/commit/2d793075a03cd2808610e50afe784210674e5651)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34688_

