Fix error messages in folly/dynamic
Fix error messages in folly/dynamic

Summary:
Some of the error messages had the incorrect expected types.
Fix those occurences

Reviewed By: shixiao

Differential Revision: D10003775

fbshipit-source-id: ba8e3b7d14ee2c5442dfd24b9fbf7775d601f6f0

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly