docs: md: fix grammar in speed_limit description

Replace 'This are' with 'These are' in the md sysfs speed limit
section to correct grammar and improve readability.

Signed-off-by: Miguel Martín Gil <miguel.martin.gil.uni@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260525214554.2196-1-miguel.martin.gil.uni@gmail.com>

Author: MigMarGil

Documentation/admin-guide/md.rst

@@ -734,7 +734,7 @@ also have
       They should be scaled by the bitmap_chunksize.
 
    sync_speed_min, sync_speed_max
-     This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
+     These are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
      however they only apply to the particular array.
 
      If no value has been written to these, or if the word ``system``