Use C17 to refer to latest C standard

See https://en.cppreference.com/w/c/17 for more information.

Author: amadio

talk/introduction/history.tex

@@ -47,7 +47,7 @@
       \node[new] (C11) [below of=C99] {C11}
       edge[direct] (C99)
       edge[transverse] (C++98);
-      \node[new] (C18) [below of=C11,node distance=1.8cm] {C18}
+      \node[new] (C17) [below of=C11,node distance=1.8cm] {C17}
       edge[direct] (C11);
       \node[new] (C++14) [below of=C++11] {\cpp14}
       edge[direct] (C++11);