docs: updated keyGenerator info

docs/pages/request-id.md

@@ -45,21 +45,20 @@ property.
 By default, it uses the `method`, `baseURL`, `params`, `data` and `url` properties from
 the request object into an hashcode generated by the `object-code` library.
 
-You can make two "different" requests share the same cache with this property.
-
-An example:
+If you need to change or share the cache for two requests based on other properties, like
+headers and etc, you can create your own Key Generator:
 
 ```js #runkit
 const axios = require('axios');
 const { setupCache, buildKeyGenerator } = require('axios-cache-interceptor');
 
-const generator = buildKeyGenerator(({ headers }) => {
+const generator = buildKeyGenerator(({ headers = {} }) => {
   // In this imaginary example, two requests will
   // be treated as the same if their x-cache-server header is the same.
 
   // The result of this function, being a object or not, will be
   // hashed by `object-code` library.
-  return headers?.['x-cache-server'] || 'not-set';
+  return headers['x-cache-server'] || 'not-set';
 });
 
 const axios = mockAxios({