Fix wrong variable name in material

[jhamberg]

In osa-4/3-listat.md the following example is given:

suurin = max(lista)
pienin = min(lista)
pituus = len(lista)

print("Pienin:", pienin)
print("Suurin:", suurin)
print("Listan pituus:", summa)

Here, the last print should use pituus instead of summa which is not defined.

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[pllk]

Kiitos!