Fix typo in exists scaladoc

[vladislavsheludchenkov]

See this scastie for reference: https://scastie.scala-lang.org/KKWe9upeRR6im3yZDfpDaQ

[mzuehlke]

Do you mind adding this as a testcase, if there isn't one already ?

[vladislavsheludchenkov]

@mzuehlke I believe this test covers it, as generator produces empty Stream sometimes. It's not explicit, but the outcome is the same: it compares it to List behavior, and List.empty.exists is always false

[mzuehlke]

Makes sense 👍