(doc) Fix small typos in the README (MODULES-11)

For https://tickets.puppetlabs.com/browse/MODULES-11

Double "the" fixed and undo a typo fix that reverted this commit:
https://github.com/puppetlabs/puppetlabs-firewall/commit/13457a4ade45f4a46d64ceb4da9d2b9582c39fcd
Merge pull request #366 from lwh/fix/master/modules-11

(doc) Fix small typos in the README (MODULES-11)