Documentation: nfsroot.rst: COSMETIC: refill a paragraph

Refill a paragraph to eliminate long lines.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/58c50f6ba94a0a2f212c4d2a42f64ffb40336b68.1578697871.git.dwlsalmeida@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>