[NFC] Correct the example in the comments of JSON.h to avoid mislead

user

Author: QingShan Zhang

llvm/include/llvm/Support/JSON.h

@@ -714,7 +714,7 @@ class ParseError : public llvm::ErrorInfo<ParseError> {
 ///         J.attribute("timestamp", int64_t(E.Time));
 ///         J.attributeArray("participants", [&] {
 ///           for (const Participant &P : E.Participants)
-///             J.string(P.toString());
+///             J.value(P.toString());
 ///         });
 ///       });
 ///   });