docs(material/slider): changed value property of mat-slider from gett…

[GuilhermeRossato]

…om getting-started guide

The mat-slider is invisible when value is 1 on the purple-green pre-built template after following the "getting started" guide. Since the white background is default nothing can be seen, which is especially bad for newbies.

This addresses a problem that causes the mat-slider from the Getting Started guide to be invisible even when perfectly following the steps, as shown below:

This is caused because the pre-build material theme purple-green.css has the following style to mat-slider's with min-value:

.mat-slider-min-value:not(.mat-slider-thumb-label-showing) .mat-slider-thumb {
    border-color: rgba(255,255,255,.3);
    background-color: transparent
}

Following the getting-started tutorial after this change yields this:

[jelbourn]

lgtm

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