CPP Client RPM sha512 links are broken

[alexandrfox]

Hey folks,

sha512 signature RPM links for cpp client (the ones here: https://pulsar.apache.org/docs/en/client-libraries-cpp/) are broken on some mirrors, e.g. a mirror link for devel RPM is leading to 404: http://ftp.tudelft.nl/apache/pulsar/pulsar-2.2.1/RPMS/apache-pulsar-client-devel-2.2.1-1.x86_64.rpm.sha512

As mentioned by @merlimat these should only be fetched from Apache servers to guarantee authenticity, so website links need to be fixed.