Fixup CWG2677: Add missing "a"

Author: eisenwave

source/basic.tex

@@ -4034,7 +4034,7 @@
 $o_1$ and $o_2$ are corresponding direct subobjects\iref{intro.object} for which:
 \begin{itemize}
 \item the complete object of $o_1$ is not const or
-\item $o_1$ is a mutable member subobject or subobject thereof.
+\item $o_1$ is a mutable member subobject or a subobject thereof.
 \end{itemize}
 \end{itemize}