fix(merge): correct typo in MergeSignature
fix(merge): correct typo in MergeSignature

MergeSignature variadic types declare types unique to each
argument.

This fixes a bug introduced in 6.3.1.

staltz/xstream,andersonleite/xstream,staltz/xstream,staltz/xstream,andersonleite/xstream,andersonleite/xstream,staltz/xstream