docs(material/chips): updates chips input accesibility recommendation

Updates example which previously used ariaPlaceholder and instead uses
an aria-label.

Author: essjay05

src/material/chips/chips.md

@@ -141,7 +141,7 @@ The chips components support 3 user interaction patterns, each with its own cont
 
 `<mat-chip-grid>` and `<mat-chip-row>` : These elements implement a grid accessibility pattern. Use them as part of a free form input that allows users to enter text to add chips.
 
-Note : be sure to have the input element be a sibling of mat-chip-grid to ensure accessibility of the input element by accessibility devices such as Voice Control.
+Note : be sure to have the input element be a sibling of mat-chip-grid to ensure accessibility of the input element by accessibility devices such as Voice Control. It is also recommended to apply an appropriate `aria-label` to the input to optimize accessibility of the input.
 
 ```html
 <mat-form-field>
@@ -159,7 +159,7 @@ Note : be sure to have the input element be a sibling of mat-chip-grid to ensure
   <input [matChipInputFor]="myChipGrid"
           [matChipInputSeparatorKeyCodes]="separatorKeysCodes"
           (matChipInputTokenEnd)="add($event)"
-          ariaPlaceholder="Add sandwich fillings..." />
+          aria-label="Add sandwich fillings..." />
 </mat-form-field>
 ```