fixup: grammer: "an" -> "a"

Co-authored-by: Jan Schultke <[email protected]>

Author: dietmarkuehl

source/exec.tex

@@ -6027,7 +6027,7 @@
 
 \pnum
 \tcode{task<T, E>} models \libconcept{sender}\iref{exec.snd}
-if \tcode{T} is \tcode{void}, a reference type, or an \cv{}-unqualified
+if \tcode{T} is \tcode{void}, a reference type, or a \cv{}-unqualified
 non-array object type and \tcode{E} is a class type. Otherwise a program
 that instantiates the definition of \tcode{task<T, E>} is ill-formed.