Fix typo in addon-contexts docs
Fix typo in addon-contexts docs

The sample code for setting per-story context settings uses the key `context`, when it should be `contexts` (presumably to match the addon's `parameterName`?).
storybooks/storybook,storybooks/storybook,kadirahq/react-storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/react-storybook,storybooks/storybook