timers: clarify lib/timer.js comment

Clarifying and using cjihrig suggestion as mentioned here:
#11018 (comment)

lib/timers.js

@@ -72,7 +72,7 @@ const TIMEOUT_MAX = 2147483647; // 2^31-1
 // timeout later, thus only needing to be appended.
 // Removal from an object-property linked list is also virtually constant-time
 // as can be seen in the lib/internal/linkedlist.js implementation.
-// Timeouts only need to process any timers due to currently timeout, which will
+// Timeouts only need to process any timers currently due to expire, which will
 // always be at the beginning of the list for reasons stated above. Any timers
 // after the first one encountered that does not yet need to timeout will also
 // always be due to timeout at a later time.