Change variable name from "$square" to "$double" or "$even" in exercise11

Minor correction in the exercise 11 notebook: I think the intended variable name for the variable `$square` is `$double` since we are doubling each value in the sequence rather than squaring it :). 

```
%%rumble
let $sequence := 1 to 10
for $value in $sequence
let $square := $value * 2  // doubling operation
where $square < 10
return $square
```