Fix typo in README
Fix typo in README

Change an underscore to a hyphen.

Change-Id: I46d3b7daaffd76456ce3f2b1aa9b215838c122a2
Reviewed-on: https://gerrit.causes.com/32321
Tested-by: jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Shane da Silva <6f6e68d1df92f30cb4b3ce35260ddf94a402f33d@causes.com>

mzp/haml-lint,skateman/haml-lint