typo in Appendix: "as Sowa [1979]a later wrote"

[Antoine-Zimmermann]

In Section A.1, there is an extra "a" after the reference to Sowa [1979] that should not be there.

[aidhog]

Fixed in HTML and PHP. Not an issue in the LaTeX. https://github.com/Knowledge-Graphs-Book/HTML-Book/commit/16e6bb49ab5058b434fd314296455c3beb540c77