Minor fixes: typos and link syntax

After [Volker Braun pointed this document out to me](https://groups.google.com/d/msg/sage-devel/ZX_k2USc7bI/gdPolaaBY6EJ), I had a look at some of it, and noticed some mistakes in passing. I hope that this pull request is an acceptable means for making these fixes available. If you'd rather have a patch on the trac server, or if I can find this documentation somewhere in the main sage git repo and use the approaches outlined in the document, please let me know.