Use past tense 'disapeared'

Author: tcharding

09_3_Testing_a_Bitcoin_Script.md

@@ -186,7 +186,7 @@ Using these commands can make it easier to see what's going on and where you are
 
 There are also a few web simulators that you can use to test scripts online. They can be superior to a command-line tool by offering a more graphical output, but we also find that they tend to have shortcomings.
 
-In the past we've tried to give extensive guidelines on using sites such as the [Script Playground](http://www.crmarsh.com/script-playground/) or the [Bitcoin Online Script Debugger](https://bitcoin-script-debugger.visvirial.com/), but they become out of date and/or disappear too quickly to keep up with them.
+In the past we've tried to give extensive guidelines on using sites such as the [Script Playground](http://www.crmarsh.com/script-playground/) or the [Bitcoin Online Script Debugger](https://bitcoin-script-debugger.visvirial.com/), but they become out of date and/or disappeared too quickly to keep up with them.
 
 Assume that these debuggers have the nice advantage of showing things visually and explicitly telling you whether a script succeeds (unlocks) or fails (stays locked). Assume that they have disadvantages with signatures, where many of them either always return `true` for signature tests or else have very cumbersome mechanisms for incorporating them.