Documentation: nfsroot.rst: Fix references to nfsroot.rst

Documentation: nfsroot.rst: Fix references to nfsroot.rst

When converting and moving nfsroot.txt to nfsroot.rst the references to
the old text file was not updated to match the change, fix this.

Fixes: 7b32f3015e7fd15a ("Documentation: nfsroot.txt: convert to ReST")
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200212181332.520545-1-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jonathan Corbet <corbet@lwn.net>