fix typo in coredns-006.md (#3168)
fix typo in coredns-006.md (#3168)

Signed-off-by: Sakura <f49b857957a4860b7ac562d17b4e800ab67560f4@daocloud.io>
johnbelamaric/coredns,coredns/coredns,miekg/coredns,johnbelamaric/coredns,yongtang/coredns,miekg/coredns,chrisohaver/coredns