docs: fix css class in custom form field control guide (#21206)
docs: fix css class in custom form field control guide (#21206)

See https://github.com/angular/components/blob/c68791db9a0b4107d04fa924c27a087cd00a8989/src/material/form-field/
form-field.ts#L323

crisbeto/material2,josephperrott/material2,mmalerba/material2,josephperrott/material2,andrewseguin/material2,mmalerba/material2,crisbeto/material2,DevVersion/material2,angular/components,andrewseguin/material2,DevVersion/material2,angular/components,angular/components,mmalerba/material2,josephperrott/material2,DevVersion/material2,angular/components,DevVersion/material2,crisbeto/material2,mmalerba/material2,crisbeto/material2,josephperrott/material2,andrewseguin/material2,andrewseguin/material2,andrewseguin/material2,DevVersion/material2,mmalerba/material2,josephperrott/material2