[meta.reflection.extract] Apply code font to "U"

Fixes NB US 111-174 (C++26 CD).

Author: jensmaurer

source/meta.tex

@@ -5935,7 +5935,7 @@
 
 \begin{itemdescr}
 \pnum
-Let $U$ be the type of the value or object that \tcode{r} represents.
+Let \tcode{U} be the type of the value or object that \tcode{r} represents.
 
 \pnum
 \returns