[DOCS] idx function is not working

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.1/static/intarray.html
Description:

 function idx(integer[], integer) does not exist
18:01:40,517 ERROR [stderr] (default task-13)   Hint: No function matches
the given name and argument types. You might need to add explicit type
casts.