doc:md: fix typo in md.txt.

   sync_speed_max

     This 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, of if the word 'system'

     If no value has been written to these, or if the word 'system'

     is written, then the system-wide value is used.  If a value,

     in kibibytes-per-second is written, then it is used.

     When the files are read, they show the currently active value