DOC: Fix statement about norms (#15050)
DOC: Fix statement about norms (#15050)

The "0.5-norm" violates the triangle inequality because its unit ball is nonconvex.
mhvk/numpy,pbrod/numpy,WarrenWeckesser/numpy,numpy/numpy,rgommers/numpy,pbrod/numpy,pbrod/numpy,mhvk/numpy,jakirkham/numpy,anntzer/numpy,mattip/numpy,WarrenWeckesser/numpy,simongibbons/numpy,madphysicist/numpy,mhvk/numpy,endolith/numpy,grlee77/numpy,WarrenWeckesser/numpy,jakirkham/numpy,charris/numpy,mattip/numpy,jakirkham/numpy,endolith/numpy,pdebuyl/numpy,WarrenWeckesser/numpy,madphysicist/numpy,numpy/numpy,jakirkham/numpy,grlee77/numpy,grlee77/numpy,grlee77/numpy,seberg/numpy,mattip/numpy,abalkin/numpy,simongibbons/numpy,anntzer/numpy,charris/numpy,grlee77/numpy,madphysicist/numpy,anntzer/numpy,charris/numpy,rgommers/numpy,simongibbons/numpy,endolith/numpy,abalkin/numpy,simongibbons/numpy,endolith/numpy,mattip/numpy,seberg/numpy,pdebuyl/numpy,numpy/numpy,pbrod/numpy,charris/numpy,pdebuyl/numpy,numpy/numpy,pdebuyl/numpy,jakirkham/numpy,madphysicist/numpy,madphysicist/numpy,mhvk/numpy,abalkin/numpy,WarrenWeckesser/numpy,seberg/numpy,pbrod/numpy,simongibbons/numpy,mhvk/numpy,rgommers/numpy,anntzer/numpy,rgommers/numpy,seberg/numpy