Update immer.d.ts

[dmorosinotto]

I changed the return type of recipe to void | S to correctly type check the use-case of producers that return new state as shown in the samples of returning data from producers in the readme docs

[coveralls]

Coverage remained the same at 96.296% when pulling 9694104 on dmorosinotto:patch_dts into 38ebb75 on mweststrate:master.

[mweststrate]

Thanks!

[dmorosinotto]

You are welcome!
Hope to see the output of new develop #128 to directly use TS in source code, so maybe in the future you can automatically produce d.ts and keep it in sync with the library 😉