docs: libs install-prefix since cmake 3.21

use the new recommendation from cmake:
  --install-prefix <directory>
     New in version 3.21.

     Specify the installation directory, used by the
     CMAKE_INSTALL_PREFIX variable. Must be an
     absolute path.

reminder: the default path is /usr/local instead of /usr

Signed-off-by: Vincent Jardin <vjardin@free.fr>

doc/developer/cross-compiling.rst

@@ -148,7 +148,7 @@ be built and installed generally like:
    CC=${HOST_ARCH}-gcc \
    CXX=${HOST_ARCH}-g++ \
    cmake \
-       -DCMAKE_INSTALL_PREFIX=/usr/${HOST_ARCH} \
+       --install-prefix /usr/${HOST_ARCH} \
        ..
    make
    make install