DOC: Fix broadcasting documentation. (#21689)
DOC: Fix broadcasting documentation. (#21689)

The example in documentation says 2d array. however it should be 3d array.

as noted in issue 21642
rgommers/numpy,mattip/numpy,rgommers/numpy,mhvk/numpy,mattip/numpy,mattip/numpy,numpy/numpy,pdebuyl/numpy,charris/numpy,mhvk/numpy,numpy/numpy,anntzer/numpy,anntzer/numpy,mattip/numpy,endolith/numpy,mhvk/numpy,pdebuyl/numpy,numpy/numpy,numpy/numpy,anntzer/numpy,endolith/numpy,pdebuyl/numpy,charris/numpy,mhvk/numpy,rgommers/numpy,charris/numpy,endolith/numpy,rgommers/numpy,pdebuyl/numpy,endolith/numpy,mhvk/numpy,charris/numpy,anntzer/numpy