DOC: fix missing random seed in optimize.newton example
DOC: fix missing random seed in optimize.newton example

This was giving occasional CI failures in the refguide check that looked like:

```
scipy.optimize.newton
---------------------
/home/travis/build/scipy/scipy/build/testenv/lib/python3.6/site-packages/scipy/optimize/zeros.py:439: RuntimeWarning: some failed to converge after 50 iterations
  warnings.warn(msg, RuntimeWarning)
File "build/testenv/lib/python3.6/site-packages/scipy/optimize/zeros.py", line 242, in newton
Failed example:
    np.allclose(vec_res, loop_res)
Expected:
    True
Got:
    False
```

Eric89GXL/scipy,matthew-brett/scipy,matthew-brett/scipy,nmayorov/scipy,e-q/scipy,jor-/scipy,tylerjereddy/scipy,arokem/scipy,zerothi/scipy,zerothi/scipy,aeklant/scipy,tylerjereddy/scipy,gertingold/scipy,ilayn/scipy,andyfaff/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,grlee77/scipy,vigna/scipy,person142/scipy,endolith/scipy,aarchiba/scipy,mdhaber/scipy,endolith/scipy,nmayorov/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,jamestwebber/scipy,ilayn/scipy,zerothi/scipy,vigna/scipy,Eric89GXL/scipy,rgommers/scipy,pizzathief/scipy,tylerjereddy/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,aeklant/scipy,lhilt/scipy,grlee77/scipy,anntzer/scipy,gertingold/scipy,zerothi/scipy,endolith/scipy,zerothi/scipy,grlee77/scipy,matthew-brett/scipy,rgommers/scipy,rgommers/scipy,e-q/scipy,aeklant/scipy,nmayorov/scipy,anntzer/scipy,mdhaber/scipy,e-q/scipy,aarchiba/scipy,gertingold/scipy,anntzer/scipy,Eric89GXL/scipy,andyfaff/scipy,arokem/scipy,matthew-brett/scipy,mdhaber/scipy,scipy/scipy,perimosocordiae/scipy,andyfaff/scipy,aarchiba/scipy,jamestwebber/scipy,pizzathief/scipy,zerothi/scipy,gertingold/scipy,endolith/scipy,gertingold/scipy,aeklant/scipy,Stefan-Endres/scipy,endolith/scipy,pizzathief/scipy,andyfaff/scipy,perimosocordiae/scipy,lhilt/scipy,WarrenWeckesser/scipy,vigna/scipy,aarchiba/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,vigna/scipy,jamestwebber/scipy,jor-/scipy,person142/scipy,andyfaff/scipy,mdhaber/scipy,ilayn/scipy,grlee77/scipy,person142/scipy,Eric89GXL/scipy,pizzathief/scipy,WarrenWeckesser/scipy,mdhaber/scipy,Eric89GXL/scipy,tylerjereddy/scipy,mdhaber/scipy,lhilt/scipy,scipy/scipy,lhilt/scipy,Stefan-Endres/scipy,anntzer/scipy,andyfaff/scipy,aarchiba/scipy,scipy/scipy,rgommers/scipy,arokem/scipy,jamestwebber/scipy,person142/scipy,jor-/scipy,ilayn/scipy,WarrenWeckesser/scipy,grlee77/scipy,lhilt/scipy,scipy/scipy,e-q/scipy,perimosocordiae/scipy,person142/scipy,aeklant/scipy,scipy/scipy,endolith/scipy,jor-/scipy,rgommers/scipy,nmayorov/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,pizzathief/scipy,vigna/scipy,ilayn/scipy,jamestwebber/scipy,anntzer/scipy,perimosocordiae/scipy,ilayn/scipy,anntzer/scipy,nmayorov/scipy,e-q/scipy,arokem/scipy,jor-/scipy,matthew-brett/scipy,arokem/scipy,scipy/scipy,perimosocordiae/scipy