Fix callback signature in README and minor test update

Author: Michal Kuffa

README.rst

@@ -173,7 +173,7 @@ send metrics based on span, result attributes.
    from datadog import statsd
    from ddtrace_graphql import patch, CLIENT_ERROR, INVALID
 
-   def callback(result=result, span=span):
+   def callback(result, span):
        tags = ['resource:{}'.format(span.resource.replace(' ', '_'))]
        statsd.increment('{}.request'.format(span.service), tags=tags)
        if span.error:

tests/test_graphql.py

@@ -296,8 +296,8 @@ def test_resource():
     @staticmethod
     def test_span_callback():
         cb_args = {}
-        def test_cb(**kwargs):
-            cb_args.update(kwargs)
+        def test_cb(result, span):
+            cb_args.update(dict(result=result, span=span))
         query = '{ hello world }'
         tracer, schema = get_traced_schema()
         result = traced_graphql(schema, query, span_callback=test_cb)