docs(button-toggle): fix missing space between HTML attrs (angular#16821)

Author: peterblazejewicz

src/dev-app/button-toggle/button-toggle-demo.html

@@ -10,7 +10,7 @@ <h1>Exclusive Selection</h1>
 
 <section>
   <mat-button-toggle-group name="alignment" [vertical]="isVertical">
-    <mat-button-toggle value="left"[disabled]="isDisabled">
+    <mat-button-toggle value="left" [disabled]="isDisabled">
       <mat-icon>format_align_left</mat-icon>
     </mat-button-toggle>
     <mat-button-toggle value="center" [disabled]="isDisabled">