fix a typo (#292)

Signed-off-by: Jimmy Tanagra <jcode@tanagra.id.au>

lib/openhab/dsl/timer_manager.rb

@@ -112,7 +112,7 @@ def reschedule(id, duration = nil)
       #
       # @example Extend an existing timer, or schedule a new one
       #   # This is technically the same functionality as just calling `after()` with an `id`,
-      #   # but allows you to performa extra steps if the timer is actually scheduled.
+      #   # but allows you to perform extra steps if the timer is actually scheduled.
       #   timers.schedule(item) do |timer|
       #     next timer.tap(&:reschedule) if timer
       #