[patch] minor spelling fix for --build --help

Binary package hint: mdadm

Spotted this trivial spelling error... patch attached.

root@upusher:~# mdadm --build --help
Usage: mdadm --build device -chunk=X --level=Y --raid-devices=Z devices

 This usage is similar to --create. The difference is that it creates
 a legacy array without a superblock. With these arrays there is no
 different between initially creating the array and subsequently
 assembling the array, except that hopefully there is useful data
 there in the second case.