Stepper playground docs use disabled nbStepperNext for disabled previous step

[MatthewJohn]

Issue type

I'm submitting a ... (check one with "x")

• [x] bug report
• [ ] feature request

Issue description

Current behavior: The playground examples contain uses of

<button nbButton disabled nbStepperNext>prev</button>

Expected behavior: This code should read as:

<button nbButton disabled nbStepperPrevious>prev</button>

Steps to reproduce: N/A

Related code:

https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-vertical.component.html#L17 https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-showcase.component.html#L12 https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-step-change-event.component.html#L14

Other information:

npm, node, OS, Browser

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Angular, Nebular

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