docs(form-field): invert "visibility" icon for password example (#13417)

Based on  https://material.io/design/components/text-fields.html#outlined-text-field

Author: AdrianPal

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

@@ -1,7 +1,7 @@
 <div class="example-container">
   <mat-form-field>
     <input matInput placeholder="Enter your password" [type]="hide ? 'password' : 'text'">
-    <mat-icon matSuffix (click)="hide = !hide">{{hide ? 'visibility' : 'visibility_off'}}</mat-icon>
+    <mat-icon matSuffix (click)="hide = !hide">{{hide ? 'visibility_off' : 'visibility'}}</mat-icon>
   </mat-form-field>
 
   <mat-form-field>