Mention `for` in the section on loops

The "4.3 Loops" section only describes `while` and `loop`. We then see `for`
used in a code sample at the end of the "13. Vectors and strings" section,
but it's explained for the first time only in the next section --
"14. Closures".

It is worth mentioning it in "4.3 Loops".
auto merge of #7001 : alco/rust/tutorial-for-loop, r=bstrie

The "4.3 Loops" section only describes `while` and `loop`. We then see `for`
used in a code sample at the end of the "13. Vectors and strings" section,
but it's explained for the first time only in the next section --
"14. Closures".

It is worth mentioning it in "4.3 Loops".

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Feel free to reword it and commit yourself if you don't like my wording.