bug(AngularMaterial/BUTTON): Correct HTML Syntax

[forsti0506]

Reproduction

Steps to reproduce:

1. Go to HTML-Code of Material Button: https://github.com/angular/components/blob/master/src/material/button/button.html
2. There you see the syntax of a material Button which leads to div tags inside a button tag which is not a valid html structure.

Fix

Replace the 2 divs with a span for valid html
<span class="mat-button-wrapper"><ng-content></ng-content></span> <span matRipple class="mat-button-ripple" [class.mat-button-ripple-round]="isRoundButton || isIconButton" [matRippleDisabled]="_isRippleDisabled()" [matRippleCentered]="isIconButton" [matRippleTrigger]="_getHostElement()"></span> <span class="mat-button-focus-overlay"></span>

[crisbeto]

There are a couple of in-progress PRs that will resolve this: #20376 and #20390.

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