Fix typo ehancement -> enhancement

While reading the docs for `Rack::Session::Encryptor` I noticed the typo.

Author: parndt

lib/rack/session/encryptor.rb

@@ -26,7 +26,7 @@ class InvalidMessage < Error
       # Options may include:
       # * :serialize_json
       #     Use JSON for message serialization instead of Marshal. This can be
-      #     viewed as a security ehancement.
+      #     viewed as a security enhancement.
       # * :pad_size
       #     Pad encrypted message data, to a multiple of this many bytes
       #     (default: 32). This can be between 2-4096 bytes, or +nil+ to disable