vim cheat sheet: added some commands

cheat-sheets/vim/vim.tex

@@ -33,6 +33,8 @@
 \begin{tabular}{@{}ll@{}}
 \keys{i}    & Insert before cursor \\
 \keys{I}    & Insert before line \\
+\keys{r}    & Replace one character \\
+\keys{R}    & Enter insert mode, but replace \\
 \end{tabular}
 
 \section{Motion}