[ML][DOCS] Fix doc error for forecast API
[ML][DOCS] Fix doc error for forecast API

Original commit: elastic/x-pack-elasticsearch@999045d5100a7ee52331817c64f9f156fa6805a6

HonzaKral/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch