doc: Fix incorrect documentation for test_custom_stats

doc: Fix incorrect documentation for test_custom_stats

The reference to the test module test_custom_stats  should have been
added under the section "Custom Cumulative Statistics", but the section
"Injection Points" has been updated instead, reversing the references
for both test modules.

d52c24b0f808 has removed a paragraph that was correct, and 31280d96a648
has added a paragraph that was incorrect.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0s4heX926+ZNh63u12gLd9jgauU6yiirKc7xGo1G01PXQ@mail.gmail.com