Fix decoding of % character for strings

Summary:
The JSONString enum member has an associated String value. The
make(a: NSObject) method attempts to convert its NSString value to a
String value by passing it to String(format:) and never provides any
arguments. Therefore it fails to decode any string that contains a
% character.

Test Plan:
Go to Math > Probability and Statistics.
Tap on the Probability Explained video.
Assert that its subtitles load.

Reviewers: laura, andy, nacho

Reviewed By: nacho

Differential Revision: https://phabricator.khanacademy.org/D15128