[tab:cpp17.destructible] Use the correct placeholder in requirement

Author: lprv

source/lib-intro.tex

@@ -1914,7 +1914,7 @@
 {p{1in}p{4.15in}}
 \topline
 \hdstyle{Expression}      &   \hdstyle{Post-condition}  \\ \capsep
-\tcode{u.\~T()} &   All resources owned by \tcode{u} are reclaimed, no exception is propagated. \\ \rowsep
+\tcode{a.\~T()} &   All resources owned by \tcode{a} are reclaimed, no exception is propagated. \\ \rowsep
 \multicolumn{2}{|l|}{
   \begin{tailnote}
   Array types and non-object types are not \oldconcept{Destructible}.