Replace `contains` with `in`

[voddan]

The examples of the contains operator are not idiomatic since they use its method form instead of its operator form:

# Checks and Actions 
## Contains

// Not idiomatic
intLi­st.c­on­tai­ns(3)  

// Is idiomatic
3 in intList 

I think the sheet will greatly benefit from replacing the methods with operators.

[Xantier]

Thanks Daniil,

Good suggestion. Added in checks to HTML.