docs: remind reader what the unit symbol means

docs/base-logic.tex

 \section{Base Logic}
 \label{sec:base-logic}
 
-The base logic is parameterized by an arbitrary CMRA $\monoid$ having a unit.
+The base logic is parameterized by an arbitrary CMRA $\monoid$ having a unit $\munit$.
 By \lemref{lem:cmra-unit-total-core}, this means that the core of $\monoid$ is a total function, so we will treat it as such in the following.
 This defines the structure of resources that can be owned.