[language/nim] confusing comment about immutable at the very start

https://learnxinyminutes.com/docs/nim/

```nim
var # Declare (and assign) variables,
letter: char = 'n' # with or without type annotations
let # Use let to declare and bind variables *once*.
legs = 400 # legs is immutable.
const # Constants are computed at compile time. This provides
debug = true # performance and is useful in compile time expressions.
```

it's very unclear to me as a complete ignorant about the language (but not ignorant about many others) what let is vs const?

to me immutable literal is no different than a constant, what am I missing? (and/or how is it different from var ?)

maybe a better example would be
```nim
let
legs = someFunction()
const
clegs = 42
```
?

Start with the Nim page at https://learnxinyminutes.com/docs/nim/ and review the opening var, let, and const example. Check Nim's distinctions between mutable variables, immutable bindings, and compile-time constants, then clarify the surrounding comments or example. Done means a newcomer can understand how let differs from const and var from this section alone.

Rédigé par le modèle d'indexation à partir du texte de l'issue.