Fixes tabs Readme selected key value

[ssidorchik]

• BREAKING CHANGE?

Description

In documentation it looks like that selectedKey value in the initialState is referencing tab label, instead of the key. The pr fixes this to use the tab key.

Before:

initialState = { selectedKey: 'Tab 1' };

After:

initialState = { selectedKey: 'tab-1' };

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 94.729% when pulling 08066b6 on ssidorchik/fix-tabs-readme into b449101 on master.