docs: proc.rst: fix indentation warning

Fix indentation snafu in proc.rst as reported by Stephen.

next-20210219/Documentation/filesystems/proc.rst:697: WARNING: Unexpected indentation.

Fixes: 93ea4a0b ("Documentation: proc.rst: add more about the 6 fields in loadavg")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210223060418.21443-1-rdunlap@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>