fixed lua_package_path in nginx.tmpl

[stibi]

I did my own build of the nginx-ingress-controller and its docker image, but I had troubles with the error_page.lua module, which couldn't be loaded, there was an error in the log, module was not found.

I think the lua package path is wrong, here is a fix.

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[k8s-reviewable]

This change is 

[stibi]

I have signed the CLA now.

[coveralls]

Coverage remained the same at 44.43% when pulling beb17f3 on stibi:patch-1 into 6ed6475 on kubernetes:master.

[thelinuxfoundation]

bump for CLA check

[aledbf]

/lgtm

[aledbf]

@stibi thanks!