DOC: correct typo in alert message about rspirep

DOC: correct typo in alert message about rspirep

This message comes when we run:
haproxy -c -V -f /etc/haproxy/haproxy.cfg
[ALERT] 072/233727 (30865) : parsing [/etc/haproxy/haproxy.cfg:34] : The 'rspirep' directive is not supported anymore sionce HAProxy 2.1. Use 'http-response replace-header' instead.
[ALERT] 072/233727 (30865) : Error(s) found in configuration file : /etc/haproxy/haproxy.cfg
[ALERT] 072/233727 (30865) : Fatal errors found in configuration.