docs: intro rule for box

docs/base-logic.tex

@@ -310,6 +310,7 @@ Furthermore, we have the usual $\eta$ and $\beta$ laws for projections, $\lambda
   {\always{\prop} \proves \always{\propB}}
 \and
 \begin{array}[c]{rMcMl}
+  \TRUE &\proves& \always{\TRUE} \\
   \always{\prop} &\proves& \prop \\
   \always{(\prop \land \propB)} &\proves& \always{(\prop * \propB)} \\
   \always{\prop} \land \propB &\proves& \always{\prop} * \propB