Another incorrect comment for pg_stat_statements

Hi, hackers

There has $subject that introduced by commit 6b4d23feef6.  When we reset the entries
if all parameters are avaiable, non-top-level entries removed first, then top-level
entries.

On branch REL_15_STABLE
Your branch is up to date with 'origin/REL_15_STABLE'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   contrib/pg_stat_statements/pg_stat_statements.c

no changes added to commit (use "git add" and/or "git commit -a")

-- 
Regrads,
Japin Li.