Updating the description to match what is in ES (#3914) (#3917)

(cherry picked from commit 79db834)

Co-authored-by: Jonathan Buttner <[email protected]>

Author: github-actions[bot]

specification/_json_spec/inference.stream_completion.json

@@ -2,7 +2,7 @@
   "inference.stream_completion": {
     "documentation": {
       "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/post-stream-inference-api.html",
-      "description": "Perform streaming inference"
+      "description": "Perform streaming completion inference"
     },
     "stability": "experimental",
     "visibility": "public",