Fix docstring unescaping

Addresses #40

prettify used the cl-format ~A directive to print strings, then
re-surrounded them with quotes. This erroneously unescaped the string.

The proper way is to use the ~S directive to print the string as an
S-expression, escaping both quotes and backslashes to survive a (read)
correctly.

refer: https://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/html/cltl/clm/node200.html#SECTION002633000000000000000