Change noSuchMethod example to be correct with null safety (and still correct in pre-nnbd code)

Author: eernstg

specification/dartLangSpec.tex

@@ -2951,7 +2951,7 @@ \subsubsection{The Method \code{noSuchMethod}}
 in order to correctly override \code{noSuchMethod} in \code{Object}.
 For instance, it can have signature
 \code{noSuchMethod(Invocation i)} or
-\code{noSuchMethod(Object i, [String s])},
+\code{noSuchMethod(Object i, [String s = ''])},
 but not
 \code{noSuchMethod(Invocation i, String s)}.
 This implies that the situation where \code{noSuchMethod} is invoked