Properly escape quotes for otp set_string example

Properly escape quotes for otp set_string example

The libss parser will consume paired double quotes, but within
a double-quoted region, repeated double quotes will be treated
as an escape and passed through as a single double quote.

(The new kadmin(1) parser in 1.14 that lets commands be specified
on the command line without -q does not go through the libss parser,
so standard shell methods for escaping quotes function as usual.)

ticket: 8469 (new)
target_version: 1.14-next
tags: pullup