[Rd] documentation for as() typo

[Paul Roebuck]

>Automatic conversion is special, and comes from the
>designer of one class of objects asserting that this
>class extends a another class (see 'setClass' and 'setIs').
              ^^^

> R.version.string
[1] "R version 2.1.1, 2005-06-20"

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SIGSIG -- signature too long (core dumped)