Fix exception message

Author: ahx

lib/openapi_first/response_parser.rb

@@ -30,7 +30,7 @@ def read_body(rack_response)
       end
       rack_response.body
     rescue TypeError
-      raise Error, "Cannot not read response body. Response should have a string-like value, but was a #{rack_response.body.class}."
+      raise Error, "Cannot not read response body. Response is not string-like, but is a #{rack_response.body.class}."
     end
 
     def build_headers_parser(headers)

spec/response_parser_spec.rb

@@ -37,7 +37,7 @@
       it 'raises an error that explains the issue' do
         expect do
           parsed.body
-        end.to raise_error 'Cannot not read response body. Response is no string-like, but is a Hash.'
+        end.to raise_error 'Cannot not read response body. Response is not string-like, but is a Hash.'
       end
     end