Adding dropForeign() example, under the dropping indexes section

dropForeign() is already mentioned in the previous section however by
adding it to the examples we clarify that it is completely parallel to
the other indexes - 'Simply concatenate the table name, the names of
the column in the index, and the index type.'

I think this adds further clarity than just having the statement - 'A
similar naming convention is used for foreign keys as is used for other
indexes' that is in an earlier section.


aside: This is my first pull request ever, so while I know its small. It means a lot to me. Hopefully I am following the proper protocol.