Update debian-mongodb.upstart.erb

Lines 18 & 22 should reference the same variable. "ENABLE_MONGOD" chosen as the proper name, as it matches what's in the 2.6.3 distribution of MongoDB on Ubuntu.
Merge pull request #323 from scole66/patch-1

Update debian-mongodb.upstart.erb
Clean: minor; corrected typos.
Merge pull request #325 from scharissis/master

Clean: minor; corrected typos.