Add {{page.root}} before /fig/... in two testing episode images

[davidbenncsiro]

Under section "Implement the third requirement":
![stack1](/fig/RPNstack42plus3.png)
should be:
![stack1]({{page.root}}/fig/RPNstack42plus3.png)

(something other than "stack1" might be good too, but doesn't really matter)

and under section "Why not add up everything?":
![42 3 7 + stack changes](/fig/RPNstack42plus3plus7.png)
should be:
![42 3 7 + stack changes]({{page.root}}/fig/RPNstack42plus3plus7.png)