Residual use of ami from #53

[sciatro]

The code group weights continue to refer to Acute MI (ami) vs the MI (mi) name change made to the codes in #53.

It looks like this is the case because 92d59b7 touched make-mapping.R only vs. 6c1dac2 which touched both make-mapping.R and make-weights.R

I just happened to notice it in passing while tracking down an unexpected result that ultimately turned out to be the sub tree grepping described in #9. It is not obvious to me that this impacts function as it seems like weights are applied by order, not name, but it's easy to imagine confusion arising.

[ellessenne]

Good catch, thanks for pointing this out - I will fix this for the next release!