Easy: Fix wrong document

Summary: VARCHAR takes 1 byte as the string length while STRING takes 4 bytes (and that is what the code shows).

The document part in the code is incorrect. Let's fix it to avoid future confusion.

Reviewed By: @chipturner

Differential Revision: D1756928

Signature: t1:1756928:1419964606:d436189bf47e16b961d6c9673e3e8c26041d9f53