Avoid using a deprecated enum case in the README (#126)
Avoid using a deprecated enum case in the README (#126)

Motivation:

The README uses the '.file' case of 'NIOSSLCertificateSource' which has
been deprecated.

Modifications:

Update the README to use the recommended replacement method:
'fromPEMFile(_:)'.

Result:

The README no longer uses a deprecated enum case.
apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl