Fixing typo in readme for varnish plugin

[anthosz]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

Thanks for the fix, could you change it to use snake_case instance_name though? Either one should work but we usually use this style in the config.

[anthosz]

@danielnelson This is now done.
Btw, all variables has been move to 'snake_case'
There are a procedure to inform the users of changes after updates or just add a point in Changelog? (with this last commit, current telegraf.conf will not works with future version of this plugin)

[danielnelson]

Sorry, I wasn't clear enough about what needed switched. We only want to use snake_case in the sample config file, but we use the normal Go style camelCase in the code.

The toml parser will deserialize instance_name into InstanceName automatically

[danielnelson]

@anthosz I was updated the docs based on this pull request in 449bd5c

[anthosz]

@danielnelson Thank you!