[docs] fixed `chain` input example

Original commit: elastic/x-pack-elasticsearch@6ac4ac4ac9

watcher/docs/reference/input/chain.asciidoc

@@ -17,9 +17,12 @@ For example, the following chain input loads data from an HTTP server using the
   "input" : {
     "chain" : {
       "inputs" : [ <1>
+        {
           "first" : {
             "simple" : { "path" : "/_search" }
+          }
         },
+        {
           "second" : {
             "http" : {
               "request" : {
@@ -29,6 +32,7 @@ For example, the following chain input loads data from an HTTP server using the
               }
             }
           }
+        }
       ]
     }
   }