Fix a callout in the field alias docs.

docs/reference/mapping/types/alias.asciidoc

@@ -16,7 +16,7 @@ PUT trips
       },
       "route_length_miles": {
         "type": "alias",
-        "path": "distance" // <1>
+        "path": "distance" <1>
       },
       "transit_mode": {
         "type": "keyword"