fix a typo

Author: chentsulin

README.md

@@ -240,7 +240,7 @@ for example:
 
 ## Additional Validation Rules
 
-GraphQL's [validation phase](https://facebook.github.io/graphql/#sec-Validation) checks the query to ensure that it can be sucessfully executed against the schema. The `validationRules` option allows for additional rules to be run during this phase. Rules are applied to each node in an AST representing the query using the Visitor pattern.
+GraphQL's [validation phase](https://facebook.github.io/graphql/#sec-Validation) checks the query to ensure that it can be successfully executed against the schema. The `validationRules` option allows for additional rules to be run during this phase. Rules are applied to each node in an AST representing the query using the Visitor pattern.
 
 A validation rule is a function which returns a visitor for one or more node Types. Below is an example of a validation preventing the specific fieldname `metadata` from being queried. For more examples see the [`specifiedRules`](https://github.com/graphql/graphql-js/tree/master/src/validation/rules) in the [graphql-js](https://github.com/graphql/graphql-js) package.