Fix typo extend -> extent in span section (#532)

Author: Qubitol

talk/morelanguage/morestl.tex

@@ -45,7 +45,7 @@
       \begin{itemize}
       \item and thus to be passed by value
       \end{itemize}
-    \item \cppinline{span} can also have a \texttt{static extend}
+    \item \cppinline{span} can also have a \texttt{static extent}
       \begin{itemize}
       \item meaning the size is determined at compile time
       \item and thus only a pointer is stored by \cppinline{span}