More cleanup.

docs/quickstart.rst

@@ -42,7 +42,7 @@ one field: ``hello`` and an input name. And when we query it, it should return `
         hello = String(argument=String(default_value="stranger"))
 
         def resolve_hello(self, info, argument):
-            return "Hello " + argument
+            return f"Hello {argument}"
 
 
     schema = Schema(query=Query)

graphene/types/field.py

@@ -41,7 +41,7 @@ def __init__(
             source and resolver
         ), "A Field cannot have a source and a resolver in at the same time."
         assert not callable(default_value), (
-            'The default value can not be a function but received "{}".'
+            'The default value cannot be a function but received "{}".'
         ).format(base_type(default_value))
 
         if required:

graphene/types/tests/test_field.py

@@ -57,7 +57,7 @@ def test_field_default_value_not_callable():
         Field(MyType, default_value=lambda: True)
     except AssertionError as e:
         # substring comparison for py 2/3 compatibility
-        assert "The default value can not be a function but received" in str(e)
+        assert "The default value cannot be a function but received" in str(e)
 
 
 def test_field_source():