tools: remove unneeded escaping in generate.js

`-` does not need to be escaped in a regular expression outside of
character classes.

PR-URL: https://github.com/nodejs/node/pull/9781


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>