DOC: Copy-edit 1.9.0-notes.rst
DOC: Copy-edit 1.9.0-notes.rst

* Fix a typo: "it's" -> "its"
* Attempt to use capitalization and punctuation more consistently.

[skip azp]

andyfaff/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,rgommers/scipy,rgommers/scipy,endolith/scipy,endolith/scipy,anntzer/scipy,Stefan-Endres/scipy,scipy/scipy,ilayn/scipy,WarrenWeckesser/scipy,ilayn/scipy,mdhaber/scipy,scipy/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,endolith/scipy,rgommers/scipy,andyfaff/scipy,endolith/scipy,anntzer/scipy,andyfaff/scipy,endolith/scipy,WarrenWeckesser/scipy,rgommers/scipy,perimosocordiae/scipy,scipy/scipy,rgommers/scipy,mdhaber/scipy,scipy/scipy,mdhaber/scipy,perimosocordiae/scipy,mdhaber/scipy,Stefan-Endres/scipy,mdhaber/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,andyfaff/scipy,Stefan-Endres/scipy,scipy/scipy,ilayn/scipy,perimosocordiae/scipy,mdhaber/scipy,anntzer/scipy,andyfaff/scipy,perimosocordiae/scipy,anntzer/scipy,scipy/scipy,ilayn/scipy,ilayn/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,anntzer/scipy,ilayn/scipy,Stefan-Endres/scipy,anntzer/scipy