[spatch] fix unparsing of metavariable containing instanceof expression

Summary: The metavariable unparser needs to add extra space before instanceof

Test Plan:
unit test included
./spatch -test

Reviewers: pieter, elynde, bkrausz

Reviewed By: pieter

CC: jcreed, bryano

Differential Revision: https://phabricator.fb.com/D1216754