plugin function name 'header' should be 'header_filter' in docs 'Custom Lua plugins'

[lidajia]

NGINX Ingress controller version:
0.33
Kubernetes version (use kubectl version):
1.18
Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

What happened:
In docs 'Custom Lua plugins', plugin function 'header' can't run in 'header_filter_by_lua' processing phases

What you expected to happen:
ngx.get_phase return 'header_filter' not 'header' on 'header_filter_by_lua' processing phases
https://github.com/openresty/lua-nginx-module#ngxget_phase

should change function name 'header' to 'header_filter'

How to reproduce it:

Anything else we need to know:

/kind bug

[aledbf]

@ElvinEfendi ping

[ElvinEfendi]

oh @agile6v beat me to it :)
I'm going to close my PR

thanks for the report @lidajia