[exec.snd.expos] Remove \tcode on indefinite article

Author: Andreas-Krug

source/exec.tex

@@ -1385,7 +1385,7 @@
 \tcode{decltype(\exposid{FWD-ENV}(declval<Env>()))}.
 
 \pnum
-For a query object \tcode{q} and \tcode{a} subexpression \tcode{v},
+For a query object \tcode{q} and a subexpression \tcode{v},
 \tcode{\exposid{MAKE-ENV}(q, v)} is an expression \tcode{env}
 whose type satisfies \exposconcept{queryable}
 such that the result of \tcode{env.query(q)} has