docs(getting-started): use var instead to match the rest of the guides

[kelvinau]

I understand that there's a PR for this similar issue already #2614, but it's been 3 months and we haven't had any updates from the last PR regarding to the const reassignment fix.

A simpler fix will be only changing docs(getting-started) to replace it let with var, as that's the only part inconsistent with the others.

[EugeneHlushko]

Hello @kelvinau
I understand how critical it is to update this example with the modern let syntax, but i would suggest we don't rush and wait two more weeks here and give original author a warning or something.

What do you think @montogeek ?

[kelvinau]

Understood. I can make the changes for let and const instead too, if @anayadesign hasn't responded after.

[montogeek]

@kelvinau Friendly ping

[kelvinau]

This was fixed by #2962 which extended #2614 already.