Update Schema class description docs.

Author: tmpfs

README.md

@@ -653,7 +653,7 @@ schema.validate(source, function(err, res) {
 function Schema(rules, [opts])
 ```
 
-Encapsulates the rules associated with a descriptor and the logic for performing validation.
+Encapsulates the rules associated with a schema and the logic for performing validation.
 
 * `rules`: The schema rules.
 * `opts`: Configuration options.

doc/readme/api.md

@@ -6,7 +6,7 @@
 function Schema(rules, [opts])
 ```
 
-Encapsulates the rules associated with a descriptor and the logic for performing validation.
+Encapsulates the rules associated with a schema and the logic for performing validation.
 
 * `rules`: The schema rules.
 * `opts`: Configuration options.