docs(button-toggle): add aria-labels to examples (#15114)

Fixes #14999

Author: Suresh918

src/material-examples/button-toggle-exclusive/button-toggle-exclusive-example.html

@@ -1,14 +1,14 @@
 <mat-button-toggle-group #group="matButtonToggleGroup">
-  <mat-button-toggle value="left">
+  <mat-button-toggle value="left" aria-label="Text align left">
     <mat-icon>format_align_left</mat-icon>
   </mat-button-toggle>
-  <mat-button-toggle value="center">
+  <mat-button-toggle value="center" aria-label="Text align center">
     <mat-icon>format_align_center</mat-icon>
   </mat-button-toggle>
-  <mat-button-toggle value="right">
+  <mat-button-toggle value="right" aria-label="Text align right">
     <mat-icon>format_align_right</mat-icon>
   </mat-button-toggle>
-  <mat-button-toggle value="justify" disabled>
+  <mat-button-toggle value="justify" disabled aria-label="Text align justify">
     <mat-icon>format_align_justify</mat-icon>
   </mat-button-toggle>
 </mat-button-toggle-group>