Minor grammar fixes in collectFields documentation

src/execution/collectFields.ts

@@ -22,9 +22,9 @@ import { typeFromAST } from '../utilities/typeFromAST';
 import { getDirectiveValues } from './values';
 
 /**
- * Given a selectionSet, collect all of the fields and returns it at the end.
+ * Given a selectionSet, collects all of the fields and returns them.
  *
- * CollectFields requires the "runtime type" of an object. For a field which
+ * CollectFields requires the "runtime type" of an object. For a field that
  * returns an Interface or Union type, the "runtime type" will be the actual
  * Object type returned by that field.
  *
@@ -52,9 +52,9 @@ export function collectFields(
 
 /**
  * Given an array of field nodes, collects all of the subfields of the passed
- * in fields, and returns it at the end.
+ * in fields, and returns them at the end.
  *
- * CollectFields requires the "return type" of an object. For a field which
+ * CollectSubFields requires the "return type" of an object. For a field that
  * returns an Interface or Union type, the "return type" will be the actual
  * Object type returned by that field.
  *