[DOCS] Fix Linkvalidator Code Example

The example code is missing the key `linktypesConfig`.

Releases: main, 13.4
Resolves: #107372
Change-Id: Icd04f7c0b8bc0f6c4d17def01452680683ba372b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/90538
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>

Author: Lukas Niestroj

typo3/sysext/linkvalidator/Documentation/Configuration/_example.tsconfig

@@ -12,8 +12,10 @@ mod.linkvalidator {
     replytoemail =
     subject = TYPO3 LinkValidator report
   }
-  external {
-    httpAgentUrl = https://example.org/info.html
-    httpAgentEmail = info@example.org
+  linktypesConfig {
+    external {
+      httpAgentUrl = https://example.org/info.html
+      httpAgentEmail = info@example.org
+    }
   }
 }