change mergeMap -> map in redux-observable example (#3794)
change mergeMap -> map in redux-observable example (#3794)

Using `mergeMap` with an inner Observable `of` is the same as using `map` by itself, so this is more idiomatic and clear.
flybayer/next.js,JeromeFitz/next.js,zeit/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,BlancheXu/test,BlancheXu/test,JeromeFitz/next.js,JeromeFitz/next.js,BlancheXu/test,flybayer/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,zeit/next.js,azukaru/next.js