Make the symmetric-enc example an example
Make the symmetric-enc example an example

Making some minor tweaks to the doc example for symmetric encryption so
it is an actual, runable example.

kimvais/cryptography,Ayrx/cryptography,Ayrx/cryptography,skeuomorf/cryptography,Hasimir/cryptography,kimvais/cryptography,skeuomorf/cryptography,Hasimir/cryptography,sholsapp/cryptography,sholsapp/cryptography,bwhmather/cryptography,bwhmather/cryptography,Ayrx/cryptography,skeuomorf/cryptography,skeuomorf/cryptography,sholsapp/cryptography,sholsapp/cryptography,kimvais/cryptography,bwhmather/cryptography,Hasimir/cryptography,Hasimir/cryptography,Ayrx/cryptography,bwhmather/cryptography,kimvais/cryptography