Changed when to once

"Once" is used once an action is complete, while "When" is used when an action begins.

Author: Lazersmoke

basic/index.html

@@ -605,7 +605,7 @@ <h2>
 
 <divclass="block">
 <p><code>git reset</code> is probably the most confusing command written
- by humans, but it can be very useful when you get the hang of it.
+ by humans, but it can be very useful once you get the hang of it.
  There are three specific invocations of it that are generally
  helpful.
 </p>