Fix typo in APIReference-Language 📖 (graphql#596)

Author: slashmo

site/graphql-js/APIReference-Language.md

@@ -196,7 +196,7 @@ function visit(root, visitor, keyMap)
 
 visit() will walk through an AST using a depth first traversal, calling
 the visitor's enter function at each node in the traversal, and calling the
-leave function after visiting that node and all of it's child nodes.
+leave function after visiting that node and all of its child nodes.
 
 By returning different values from the enter and leave functions, the
 behavior of the visitor can be altered, including skipping over a sub-tree of