Improve grammar a little at zpoolconcepts.7

Signed-off-by: Andriy Tkachuk <andriy.tkachuk@seagate.com>

Author: andriytk

man/man7/zpoolconcepts.7

@@ -174,8 +174,8 @@ draid3 - three enclosures, for each group it will look as if only one,
 two or three disks are failed.
 .It Ar spares
 The number of distributed hot spares.
-If failure groups are configured (width > children), it should be
-multiple of the number of such groups so that each group have the same
+If failure groups are configured (width > children), it must be a
+multiple of the number of groups so that each group has the same
 number of spares.
 Defaults to zero.
 .El