[docs] fix typo in file README_COAP.md (#3972)
[docs] fix typo in file README_COAP.md (#3972)


srickardti/openthread,jwhui/openthread,lanyuwen/openthread,jwhui/openthread,librasungirl/openthread,openthread/openthread,turon/openthread,lanyuwen/openthread,srickardti/openthread,chshu/openthread,lanyuwen/openthread,abtink/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,turon/openthread,bukepo/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,chshu/openthread,chshu/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,jwhui/openthread,librasungirl/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,turon/openthread,chshu/openthread,openthread/openthread,chshu/openthread,openthread/openthread,turon/openthread