mergeMapConcurrently flow type has reversed parameters

[briancavalier]

The flow type of mergeMapConcurrently list the initial parameters as concurrency, f, but the implementation says f, concurrency.

See also #288, which adds the missing TypeScript defs with the correct parameter order (matching the implementation).

For 1.x, let's fix the flow type.

It may be nice to consider a future (breaking) change to reverse the parameters in the implementation, since partially applying concurrency would result in the same type as other higher order combinators, like chain.