fix(https_x509_bundle): replace unreliable external URL in https_x509_bundle example

Replace howsmyssl.com with letsencrypt.org in the https_x509_bundle
example. howsmyssl.com is a third-party server that is frequently
unreachable from CI, causing flaky test failures. letsencrypt.org
chains to the same ISRG Root X1 CA, so the custom certificate bundle
validation coverage is identical.

(cherry picked from commit d6596eff3a)

examples/protocols/https_x509_bundle/main/https_x509_bundle_example_main.c

@@ -1,7 +1,7 @@
 /* HTTPS GET Example using plain mbedTLS sockets
  *
- * Contacts the howsmyssl.com API via TLS v1.2 and reads a JSON
- * response.
+ * Connects to multiple HTTPS servers and validates their certificates
+ * using the certificate bundle.
  *
  * Adapted from the ssl_client1 example in mbedtls.
  *
@@ -46,7 +46,7 @@
 #define MAX_URLS    2
 
 static const char *web_urls[MAX_URLS] = {
-    "https://www.howsmyssl.com/a/check",
+    "https://letsencrypt.org",
     "https://espressif.com",
 };