Stepper playground docs use disabled nbStepperNext for disabled previous step

[MatthewJohn]

Issue type

I'm submitting a ... (check one with "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:

nebular/src/playground/with-layout/stepper/stepper-vertical.component.html

Line 17 in 4c4d846

<button nbButton disabled nbStepperNext>prev</button>

nebular/src/playground/with-layout/stepper/stepper-showcase.component.html

Line 12 in 4c4d846

<button nbButton disabled nbStepperNext>prev</button>

nebular/src/playground/with-layout/stepper/stepper-step-change-event.component.html

Line 14 in 4c4d846

<button nbButton disabled nbStepperNext>prev</button>

Other information:

npm, node, OS, Browser

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Angular, Nebular

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