doc: small grammar fix in cli/npm.md
doc: small grammar fix in cli/npm.md

Reviewed-By: da39a3ee5e6b4b0d3255bfef95601890afd80709@zkat
Credit: da39a3ee5e6b4b0d3255bfef95601890afd80709@andresilveira
PR-URL: https://github.com/npm/npm/pull/12802

DaveEmmerson/npm,evocateur/npm,Volune/npm,DaveEmmerson/npm,Volune/npm,evocateur/npm,chadnickbok/npm,misterbyrne/npm,Volune/npm,evocateur/npm,misterbyrne/npm,DaveEmmerson/npm,misterbyrne/npm,chadnickbok/npm,chadnickbok/npm