docs: fix example in ctx.md

[Abdallah-Zidan]

fix check_beta_user function parameter type

[eaufavor]

Both are correct and our current doc in the doc matches the code here.

[Abdallah-Zidan]

Thanks for clarification I think following the guide failed with me because in the shared example Cargo.toml here there is this line

pingora-http = { version = "0.1.0", path = "../pingora-http" } 

but I followed the guide to use these dependencies

[dependencies]
async-trait="0.1"
pingora = { version = "0.1", features = [ "lb" ] }