Fix cardinality of master thesis authors

A master thesis should only have one author, as far as I’m aware, and
there are no counterexamples on Wikidata [1].

[1]: https://query.wikidata.org/#SELECT%20%3Fthesis%20WHERE%20%7B%0A%20%20%3Fthesis%20wdt%3AP31%2Fwdt%3AP279%2a%20wd%3AQ1907875%3B%0A%20%20%20%20%20%20%20%20%20%20wdt%3AP50%20%3Fauthor1%2C%20%3Fauthor2.%0A%20%20FILTER%28%3Fauthor1%20%21%3D%20%3Fauthor2%29%0A%7D

presentation/slides.tex

@@ -44,7 +44,7 @@
     \begin{lstlisting}[language=sparql]
 <master_thesis> {
   wdt:P1476 xsd:langString; # title
-  wdt:P50 @<human>+ # author
+  wdt:P50 @<human> # author
 }
 
 <human> {