[encoding] Clarify result of division in algorithms (#20)

where an algorithm tells you to do something like

```Let lead be pointer / 157 + 0x81. ```

it's not immediately clear that you need to use the floor function, rather than rounding, to bring the result of the division back to an integer.  It would be good to make this clearer in the text.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/encoding/issues/20