fix linting in code example

Author: MrTango

docs/backend/fields.md

@@ -199,9 +199,9 @@ class ITestSchema(model.Schema):
 
     body = RichText(
         title="Body text",
-        default_mime_type='text/x-rst',
-        output_mime_type='text/x-html',
-        allowed_mime_types=('text/x-rst', 'text/structured',),
+        default_mime_type="text/x-rst",
+        output_mime_type="text/x-html",
+        allowed_mime_types=("text/x-rst", "text/structured",),
         default=defaultBody,
     )
 ```