Fix "arugment" typo in README

Hi,

thanks for the package. While going through the readme I noticed one "argument" was spelled wrong on one occasion.

BR,
Marten

Author: schiwekM

README.md

@@ -37,7 +37,7 @@ proxyaddr(req, function (addr) { return addr === '127.0.0.1' })
 proxyaddr(req, function (addr, i) { return i < 1 })
 ```
 
-The `trust` arugment may also be a single IP address string or an
+The `trust` argument may also be a single IP address string or an
 array of trusted addresses, as plain IP addresses, CIDR-formatted
 strings, or IP/netmask strings.