C++: Change the id of the experimental double-free query to not overlap with the new non-experimental one.

Author: MathiasVP

cpp/ql/src/experimental/Security/CWE/CWE-415/DoubleFree.ql

@@ -2,7 +2,7 @@
  * @name Errors When Double Free
  * @description Freeing a previously allocated resource twice can lead to various vulnerabilities in the program.
  * @kind problem
- * @id cpp/double-free
+ * @id cpp/experimental-double-free
  * @problem.severity warning
  * @precision medium
  * @tags security