[meta.reflection.layout] Spell variable "T", not "TR".

It's unclear whether this was just a typo or an attempt to avoid
ambiguity, but there does not seem to be a risk of confusion, and the
spelling of the paper ("T") seems fine.

Author: tkoeppe

source/meta.tex

@@ -5771,7 +5771,7 @@
   within any object of type \tcode{parent_of(r)}.
 \item
   Otherwise, \tcode{r} represents a data member description
-  $(\mathit{TR}, N, A, W, \mathit{NUA})$\iref{class.mem.general}.
+  $(T, N, A, W, \mathit{NUA})$\iref{class.mem.general}.
   If $A$ is not $\bot$,
   then the value of $A$.
   Otherwise, \tcode{alignment_of(\reflexpr{$T$})}.