Update PDF cheatsheet with new Rust bindings

doc/prelude-cheatsheet.tex

@@ -337,6 +337,8 @@
   \subsection{prelude-rust}
   \begin{keylist}
   \item[C-c C-d] racer-describe
+  \item[C-c .] racer-find-definition
+  \item[C-c ,] pop-tag-mark
   \end{keylist}
 
 \end{multicols}