doc: bitbake-user-manual: add explicit target for crates fetcher
    
    Like in other sections describing fetchers
    
    Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
    Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>