Set correct init_file block header

This is a matchsubs block, ensure to set the correct header for
atmsort.pm parsing.

src/pl/plpython/init_file

--- start_matchignore
+-- start_matchsubs
 m/\(plpython\.c:\d+\)/
 s/\(plpython\.c:\d+\)//
 -- end_matchsubs