doc: module function instead of require

[decadence]

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.

Isn't module an old form of requiring modules?

[lynch1981]

You can add doc: prefix to the title to fix Lint error ;)

[zhuizhuhaomeng]

@decadence where did you get the old style?
From the stackoverflow, the original doc is right.
https://stackoverflow.com/questions/7248877/in-lua-how-do-you-import-modules

[decadence]

@zhuizhuhaomeng it is listed in the text after that:

The Lua module is actually loaded by the first Nginx request handler and is cached by the require() built-in in the package.loaded table for later reference, and the module() builtin used by some Lua modules has the side effect of setting a global variable to the loaded module table

Yes, I think I got it mixed up. module is used inside the module file itself, and not when module is required.