CheckSpecVersion reference fix (#8056)

* CheckSpecVersion reference fix

* Update frame/example/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/frame/example/src/lib.rs

@@ -577,7 +577,8 @@ impl<T: Config> Module<T> {
 //
 // Note that a signed extension can also indicate that a particular data must be present in the
 // _signing payload_ of a transaction by providing an implementation for the `additional_signed`
-// method. This example will not cover this type of extension. See `CheckRuntime` in FRAME System
+// method. This example will not cover this type of extension. See `CheckSpecVersion` in 
+// [FRAME System](https://github.com/paritytech/substrate/tree/master/frame/system#signed-extensions)
 // for an example.
 //
 // Using the extension, you can add some hooks to the life cycle of each transaction. Note that by