bus_if: Fix typo

Reviewed by:	olce
Approved by:	olce
Fixes:	4c4392e791 ("Add doxygen doc comments for most of newbus and the BUS interface.")
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D48721

sys/kern/bus_if.m

@@ -31,7 +31,7 @@
 
 /**
  * @defgroup BUS bus - KObj methods for drivers of devices with children
- * @brief A set of methods required device drivers that support
+ * @brief A set of methods required for device drivers that support
  * child devices.
  * @{
  */