FetchContent: Fix documentation for example usage
The protobuf example would raise a "Policy CMP0077" warning and protobuf tests would stil be built, e.g. setting the variable is ignored in latest CMake.
Thus, cache the protobuf_BUILD_TESTS variable so the example works independent of CMP0077.
Topic-rename: FetchContent-protobuf-example-CMP0077
Edited by Craig Scott