Update README

README.md

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 # Proxy Scheme, Host and Port detection middleware
 
-PSR-7 Middleware that determines the scheme, host and port from the 'X-Forwarded-Proto', 'X-Forwarded-Host' and 'X-Forwarded-Port' headers and updates the Request's Uri object.
+PSR-15 Middleware that determines the scheme, host and port from the 'X-Forwarded-Proto', 'X-Forwarded-Host' and 'X-Forwarded-Port' headers and updates the Request's Uri object.
 
 You can set a list of proxies that are trusted as the second constructor parameter. If this list is set, then the proxy headers will only be checked if the `REMOTE_ADDR` is in the trusted list.