docs: improve type safety in custom validator documentation

Replace `field: any` with proper SchemaPath typing in Signal Forms custom
validation examples to match built-in validator patterns.

fixes: angular#66887

Update adev/src/content/guide/forms/signals/validation.md

Co-authored-by: Matthieu Riegler <kyro38@gmail.com>

Author: MeAkib

adev/src/content/guide/forms/signals/validation.md

@@ -495,8 +495,8 @@ Return an error object with `kind` and `message` when validation fails. Return `
 Create reusable validation rule functions by wrapping `validate()`:
 
 ```ts
-function url(field: any, options?: {message?: string}) {
-  validate(field, ({value}) => {
+function url(path: SchemaPath<string>, options?: {message?: string}) {
+  validate(path, ({value}) => {
     try {
       new URL(value());
       return null;
@@ -509,8 +509,8 @@ function url(field: any, options?: {message?: string}) {
   });
 }
 
-function phoneNumber(field: any, options?: {message?: string}) {
-  validate(field, ({value}) => {
+function phoneNumber(path: SchemaPath<string>, options?: {message?: string}) {
+  validate(path, ({value}) => {
     const phoneRegex = /^\d{3}-\d{3}-\d{4}$/;
 
     if (!phoneRegex.test(value())) {