Fix wording for missing model

(cherry picked from commit 57fa0ae)

Author: scottgonzalez

lib/jsonapi/resource.rb

@@ -1001,7 +1001,7 @@ def _model_class
 
         @model_class = model_name.to_s.safe_constantize
         if @model_class.nil?
-          warn "[MODEL NOT FOUND] Model could not be found for #{self.name}. If this a base Resource declare it as abstract."
+          warn "[MODEL NOT FOUND] Model could not be found for #{self.name}. If this is a base Resource declare it as abstract."
         end
 
         @model_class

test/unit/resource/resource_test.rb

@@ -184,7 +184,7 @@ def test_nil_model_class
     # ToDo:Figure out why this test does not work on Rails 4.0
     # :nocov:
     if (Rails::VERSION::MAJOR >= 4 && Rails::VERSION::MINOR >= 1) || (Rails::VERSION::MAJOR >= 5)
-      assert_output nil, "[MODEL NOT FOUND] Model could not be found for NoMatchResource. If this a base Resource declare it as abstract.\n" do
+      assert_output nil, "[MODEL NOT FOUND] Model could not be found for NoMatchResource. If this is a base Resource declare it as abstract.\n" do
         assert_nil NoMatchResource._model_class
       end
     end
@@ -593,7 +593,7 @@ class NoModelResource < JSONAPI::Resource
         NoModelResource._model_class
       CODE
     end
-    assert_match "[MODEL NOT FOUND] Model could not be found for ResourceTest::NoModelResource. If this a base Resource declare it as abstract.\n", err
+    assert_match "[MODEL NOT FOUND] Model could not be found for ResourceTest::NoModelResource. If this is a base Resource declare it as abstract.\n", err
   end
 
   def test_no_warning_when_abstract