Unify usage hint formatting

[dg0yt]

Follow pattern from CMake packages. In particular, indent with append_indent().

Fixes mixed indentation when a port has both CMake and pkg-config (and copied like that into usage files, e.g.):

libosdp provides CMake targets:

  find_package(LibOSDP CONFIG REQUIRED)
  target_link_libraries(main PRIVATE $<IF:$<TARGET_EXISTS:libosdp::osdp>,libosdp::osdp,libosdp::osdpstatic>)

libosdp provides pkg-config modules:

    # Open Supervised Device Protocol (OSDP) Library
    libosdp

 ^^^

[BillyONeal]

Thanks for the fix!