docs(chips): add aria-labels to examples (#15167)

Fixes #15010

src/material-examples/chips-autocomplete/chips-autocomplete-example.html

@@ -1,5 +1,5 @@
 <mat-form-field class="example-chip-list">
-  <mat-chip-list #chipList>
+  <mat-chip-list #chipList aria-label="Fruit selection">
     <mat-chip
       *ngFor="let fruit of fruits"
       [selectable]="selectable"

src/material-examples/chips-input/chips-input-example.html

@@ -1,5 +1,5 @@
 <mat-form-field class="example-chip-list">
-  <mat-chip-list #chipList>
+  <mat-chip-list #chipList aria-label="Fruit selection">
     <mat-chip *ngFor="let fruit of fruits" [selectable]="selectable"
              [removable]="removable" (removed)="remove(fruit)">
       {{fruit.name}}

src/material-examples/chips-overview/chips-overview-example.html

@@ -1,4 +1,4 @@
-<mat-chip-list>
+<mat-chip-list aria-label="Fish selection">
   <mat-chip>One fish</mat-chip>
   <mat-chip>Two fish</mat-chip>
   <mat-chip color="primary" selected>Primary fish</mat-chip>

src/material-examples/chips-stacked/chips-stacked-example.html

@@ -1,4 +1,4 @@
-<mat-chip-list class="mat-chip-list-stacked">
+<mat-chip-list class="mat-chip-list-stacked" aria-label="Color selection">
   <mat-chip *ngFor="let chip of availableColors" selected [color]="chip.color">
     {{chip.name}}
   </mat-chip>