[dcl.attr.contract] Fix sentence defining violation handler

Author: jensmaurer

source/declarations.tex

@@ -4591,7 +4591,7 @@
 The \defn{violation handler} of a program is a function of type
 ``\opt{\tcode{noexcept}} function of
 (lvalue reference to \tcode{const std::contract_violation})
-returning \tcode{void}'' specified in an
+returning \tcode{void}'', and is specified in an
 \impldef{specification of violation handler} manner.
 The violation handler is invoked
 when the predicate of a checked contract evaluates to \tcode{false}