Improve explanation of "Where" in Some Primitive Functions chapter.

@mbaas2 reports via email:

> “in the explanation of rules for computation of haystack⍳needles, you wrote
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> "- items which do not appear in `haystack` give the result `1+m` if `m` is the leading number in the shape of `haystack`, i.e. `m ← ≢haystack`."
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> I find that confusing, because you speak of "shape" and then use ≢ - and that will never return more than 1 number, so the special emphasis on a leading number provoques unneccessary questions, I think ;)
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> I'd like to suggest an alternative:
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> "- items which do not appear in `haystack` give the result `1+m` if `m` is the number of element of `haystack`, i.e. `m ← ≢haystack`."