Fix keyword in note of lesson 11 chapter 6

[Schrubitteflau]

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• I didn't use a machine to do these translations
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Change the keyword from var to const in the note of lesson 11 chapter 6. It says :

Note: We want contractInstance to be limited in scope to the block in which it's defined. Use let instead of var.

But we're using const, not var, so the correct note should be :

Note: We want contractInstance to be limited in scope to the block in which it's defined. Use let instead of var.