Fixes #8198 KStreams testing docs use non-existent method pipe (#6678)
Fixes #8198 KStreams testing docs use non-existent method pipe (#6678)

Minor fix of #8198 apache/kafka-site#210

Reviewers: John Roesler <a51dda7c7ff50b61eaea0444371f4a6a9301e501@confluent.io>, Bill Bejeck <bbejeck@gmail.com>
guozhangwang/kafka,noslowerdna/kafka,lindong28/kafka,TiVo/kafka,guozhangwang/kafka,Chasego/kafka,noslowerdna/kafka,noslowerdna/kafka,TiVo/kafka,TiVo/kafka,guozhangwang/kafka,sslavic/kafka,apache/kafka,lindong28/kafka,guozhangwang/kafka,sslavic/kafka,apache/kafka,apache/kafka,lindong28/kafka,Chasego/kafka,lindong28/kafka,sslavic/kafka,Chasego/kafka,sslavic/kafka,Chasego/kafka,noslowerdna/kafka,apache/kafka,TiVo/kafka