drivers/clock: Fix comment

Fixed error in ::clock's Doxygen comment. It referred to
osv::clock::monotonic, while in fact the correct name is osv::clock::uptime.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

drivers/clock.hh

@@ -24,7 +24,7 @@
  * The methods of this class are not type-safe, in that they return times
  * as unadorned integers (s64) whose type does not specify the time units
  * or the epoch. Prefer instead to use the types from <osv/clock.hh>:
- * osv::clock::monotonic et al. Their efficiency is identical to the
+ * osv::clock::uptime et al. Their efficiency is identical to the
  * methods of this class, but they allow much better compile-time checking.
  */
 class clock {