Replace var with let

[IvanBila]

Summary

I've noticed that var is currently used in some parts of the firestore documentation over of let or const.
Using var is highly discouraged because:

1. No error is thrown if you declare the same variable twice using var;
2. Variables declared with var are not block-scoped(except some cases if they are in functions), they might even cause variable name collision.

On the other hand const and let allow block scoping and immutable variables(via reassignment).

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[samtstern]

@IvanBila thanks! Will merge once the CLA is worked out.

[IvanBila]

@samtstern I've Signed the CLA

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[samtstern]

Thank you @IvanBila for the contributioN!