docs(material/form-field): change prefix example to use icon for better alignment

Author: annieyw

src/components-examples/material/form-field/form-field-prefix-suffix/form-field-prefix-suffix-example.css

@@ -2,15 +2,7 @@
   margin-left: 8px;
 }
 
-.example-right-align {
-  text-align: right;
-}
-
 input.example-right-align::-webkit-outer-spin-button,
 input.example-right-align::-webkit-inner-spin-button {
   display: none;
 }
-
-input.example-right-align {
-  -moz-appearance: textfield;
-}

src/components-examples/material/form-field/form-field-prefix-suffix/form-field-prefix-suffix-example.html

@@ -8,9 +8,8 @@
   </mat-form-field>
 
   <mat-form-field appearance="fill">
-    <mat-label>Amount</mat-label>
-    <input matInput type="number" class="example-right-align">
-    <span matPrefix>$&nbsp;</span>
-    <span matSuffix>.00</span>
+    <mat-label>Network ID</mat-label>
+    <input matInput type="text">
+    <mat-icon matPrefix>wifi</mat-icon>
   </mat-form-field>
 </div>