Add missing punctuation.

CppCoreGuidelines.md

@@ -2973,7 +2973,7 @@ These operations disagree about copy semantics. This will lead to confusion and
 
 ## <a name="SS-dtor"></a> C.dtor: Destructors
 
-Does this class need a destructor is a surprisingly powerful design question.
+"Does this class need a destructor?" is a surprisingly powerful design question.
 For most classes the answer is "no" either because the class holds no resources or because destruction is handled by [the rule of zero](#Rc-zero);
 that is, its members can take care of themselves as concerns destruction.
 If the answer is "yes", much of the design of the class follows (see [the rule of five](#Rc-five).