Change undeclared variable hint to `let` from `var (#2119)
Change undeclared variable hint to `let` from `var (#2119)

We now teach `let` instead of `var` in the Intro curriculum, so this suggestion seems more appropriate.

Co-authored-by: Mat Brown <67d3ec7203045c37cb313e41ac5420fb4abc80e8@fastmail.com>

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