fix misspelling of the word optional in the error message (#397)

Author: mojavelinux

packages/convict/src/main.js

@@ -614,7 +614,7 @@ const convict = function convict(def, opts) {
       options.allowed = options.allowed || ALLOWED_OPTION_WARN
 
       if (options.output && typeof options.output !== 'function') {
-        throw new Error('options.output is optionnal and must be a function.')
+        throw new Error('options.output is optional and must be a function.')
       }
 
       const output_function = options.output || global.console.log

packages/convict/test/validation.test.js

@@ -131,7 +131,7 @@ describe('configuration files contain properties not declared in the schema', fu
       config.validate({
         output: 312
       })
-    }).toThrow(/options\.output is optionnal and must be a function\./)
+    }).toThrow(/options\.output is optional and must be a function\./)
   })
 
   test('must not break on consecutive overrides', function() {