[plt-scheme] Possible mistake in the documentation of call/ec

Hello,

the documentation of PLT Scheme 4.1.2, section 9.4, states that the
procedure call-with-escape-continuation and its alias call/ec take one
or two arguments.

However, an experiment at the REPL shows that call/ec accepts only one
argument. As far as I understand, it would also be rather pointless to
specify a delimiting prompt tag for an escape continuation.

Therefore I think the procedure behaves correctly and this is probably a
small mistake in the documentation.

cu,
Thomas