doc: nrf-bm: install_nrf_bm: update directory structure

Updates the directory structure to reflect the installation
process.

Signed-off-by: Mirko Covizzi <[email protected]>

Author: MirkoCovizzi

doc/nrf-bm/install_nrf_bm.rst

@@ -125,7 +125,7 @@ Your directory structure should now look similar to this:
    ncs
    ├─── toolchains
    │  └─── <toolchain_version>
-   └─── bm_<sdk_version>
+   └─── nrf-bm/<sdk_version>
       ├─── bootloader
       ├─── modules
       ├─── nrf