[expr.const.const] Fix "subobject" typo (cplusplus#9357)

Author: mpolacek

source/expressions.tex

@@ -9131,7 +9131,7 @@
 or a subobject thereof,
 \item
 a template parameter object\iref{temp.param}
-or a subject thereof, or
+or a subobject thereof, or
 \item
 an object whose lifetime begins and ends
 during the evaluation of a core constant expression.