MINOR: Streams input topic corrected (#6513)
MINOR: Streams input topic corrected (#6513)

Though out the tutorial, the name of the input topic that was created is `streams-plaintext-input`. However, this was mistaken at some point in the tutorial and changed to `streams-wordcount-input`.

This patch is to adjust that. Thanks.

Reviewers: Guozhang Wang <fae442ba222671b2ebdf7511c9ca77079c43031b@gmail.com>
guozhangwang/kafka,noslowerdna/kafka,sslavic/kafka,sslavic/kafka,KevinLiLu/kafka,KevinLiLu/kafka,noslowerdna/kafka,TiVo/kafka,lindong28/kafka,Chasego/kafka,Chasego/kafka,sslavic/kafka,guozhangwang/kafka,Chasego/kafka,guozhangwang/kafka,apache/kafka,lindong28/kafka,noslowerdna/kafka,lindong28/kafka,guozhangwang/kafka,Chasego/kafka,KevinLiLu/kafka,noslowerdna/kafka,apache/kafka,apache/kafka,TiVo/kafka,KevinLiLu/kafka,apache/kafka,sslavic/kafka,lindong28/kafka,TiVo/kafka,TiVo/kafka