Fix incorrect example for disabling promql/aggregate

In the definition on how to disable the promql/aggregate rule
for `strip` labels, it is stated to use :false as the suffix,
but then uses :true in the example with a specific label.

Update the second example to match the first, correct, one.

docs/checks/promql/aggregate.md

@@ -123,7 +123,7 @@ Example:
 Example:
 
 ```yaml
-# pint disable promql/aggregate(instance:true)
+# pint disable promql/aggregate(instance:false)
 ```
 
 ## How to snooze it