dns: fix ReferenceError in resolve() error path

A typo in the variable name makes it throw a ReferenceError instead of
the expected "Unknown type" error when dns.resolve() is passed a bad
record type argument.

Fixes the following exception:

  ReferenceError: type is not defined
    at Object.exports.resolve (dns.js:189:40)
    at /Users/bnoordhuis/src/master/test/simple/test-c-ares.js:48:9
    <snip>