Remove unneeded escape from 'after\_filter'

Presently, the affected text renders as 'after\_filter' on GitHub:
https://github.com/tmeasday/meteor-router#filtering

GitHub Flavored Markdown modifies standard markdown's handling of underscores,
negating the need to escape underscores appearing in words or code blocks.
For details on these modifications, check out the "Code" section toward the
end of the GFM docs:
https://help.github.com/articles/github-flavored-markdown