[basic.link] Use maths font in defn of direct base class relationship (#8333)

Fixes NB US 1-405 (C++26 CD).

Author: eisenwave

source/basic.tex

@@ -3268,10 +3268,10 @@
 \item
 an entity, value, or object that is TU-local,
 \item
-a direct base class relationship (\tcode{D}, \tcode{B})\iref{class.derived.general}
-for which either \tcode{D} or \tcode{B} is TU-local, or
+a direct base class relationship $(D, B)$\iref{class.derived.general}
+for which either $D$ or $B$ is TU-local, or
 \item
-a data member description ($T$, $N$, $A$, $W$, $\mathit{NUA}$)\iref{class.mem.general}
+a data member description $(T, N, A, W, \mathit{NUA})$\iref{class.mem.general}
 for which $T$ is TU-local.
 \end{itemize}
 \end{itemize}