docs($http): fix a typo (his --> this)

Closes #14430

src/ng/http.js

@@ -555,7 +555,7 @@ function $HttpProvider() {
      * That means changes to the properties of `data` are not local to the transform function (since Javascript passes objects by reference).
      * For example, when calling `$http.get(url, $scope.myObject)`, modifications to the object's properties in a transformRequest
      * function will be reflected on the scope and in any templates where the object is data-bound.
-     * To prevent his, transform functions should have no side-effects.
+     * To prevent this, transform functions should have no side-effects.
      * If you need to modify properties, it is recommended to make a copy of the data, or create new object to return.
      * </div>
      *