DOC: fix typo in vectorize docstring
DOC: fix typo in vectorize docstring

"an single" typo pointed out by keithbriggs - see gh-11730.

pbrod/numpy,jakirkham/numpy,jorisvandenbossche/numpy,anntzer/numpy,pbrod/numpy,mhvk/numpy,grlee77/numpy,MSeifert04/numpy,seberg/numpy,madphysicist/numpy,pizzathief/numpy,seberg/numpy,shoyer/numpy,numpy/numpy,jorisvandenbossche/numpy,endolith/numpy,pizzathief/numpy,shoyer/numpy,pbrod/numpy,shoyer/numpy,anntzer/numpy,simongibbons/numpy,mhvk/numpy,grlee77/numpy,pbrod/numpy,endolith/numpy,ahaldane/numpy,pizzathief/numpy,gfyoung/numpy,jakirkham/numpy,endolith/numpy,grlee77/numpy,ahaldane/numpy,pdebuyl/numpy,MSeifert04/numpy,abalkin/numpy,jakirkham/numpy,rgommers/numpy,gfyoung/numpy,numpy/numpy,jakirkham/numpy,ahaldane/numpy,charris/numpy,madphysicist/numpy,endolith/numpy,jorisvandenbossche/numpy,numpy/numpy,jorisvandenbossche/numpy,mattip/numpy,mhvk/numpy,numpy/numpy,rgommers/numpy,pizzathief/numpy,pbrod/numpy,pdebuyl/numpy,madphysicist/numpy,anntzer/numpy,jorisvandenbossche/numpy,mattip/numpy,seberg/numpy,MSeifert04/numpy,abalkin/numpy,pdebuyl/numpy,shoyer/numpy,MSeifert04/numpy,madphysicist/numpy,simongibbons/numpy,ahaldane/numpy,rgommers/numpy,pdebuyl/numpy,MSeifert04/numpy,gfyoung/numpy,abalkin/numpy,WarrenWeckesser/numpy,madphysicist/numpy,WarrenWeckesser/numpy,pizzathief/numpy,simongibbons/numpy,mattip/numpy,WarrenWeckesser/numpy,mhvk/numpy,WarrenWeckesser/numpy,charris/numpy,grlee77/numpy,anntzer/numpy,grlee77/numpy,mattip/numpy,shoyer/numpy,rgommers/numpy,jakirkham/numpy,charris/numpy,ahaldane/numpy,simongibbons/numpy,simongibbons/numpy,seberg/numpy,WarrenWeckesser/numpy,charris/numpy,mhvk/numpy