Grammar fixes;

Author: bjornbytes

spec/Section 5 -- Validation.md

@@ -870,7 +870,7 @@ fragment ownerFragment on Dog {
 
 ** Formal Specification **
 
-  * For each {spread} (named or inline) in defined in the document.
+  * For each {spread} (named or inline) defined in the document.
   * Let {fragment} be the target of {spread}
   * Let {fragmentType} be the type condition of {fragment}
   * Let {parentType} be the type of the selection set containing {spread}
@@ -893,7 +893,7 @@ the parent type.
 
 ##### Object Spreads In Object Scope
 
-In the scope of a object type, the only valid object type
+In the scope of an object type, the only valid object type
 fragment spread is one that applies to the same type that
 is in scope.
 
@@ -1100,8 +1100,8 @@ usage of a directive, the directive must be available on that server.
 
 ** Explanatory Text **
 
-Variable defined by operations are allowed to define default values
-if the type of that variable not non-null.
+Variables defined by operations are allowed to define default values
+if the type of that variable is not non-null.
 
 For example the following query will pass validation.