Improve strings in openai_conversation.generate_image action (#139736)

Use descriptive wording, fix sentence-casing.

homeassistant/components/openai_conversation/strings.json

@@ -48,10 +48,10 @@
   "services": {
     "generate_image": {
       "name": "Generate image",
-      "description": "Turn a prompt into an image",
+      "description": "Turns a prompt into an image",
       "fields": {
         "config_entry": {
-          "name": "Config Entry",
+          "name": "Config entry",
           "description": "The config entry to use for this action"
         },
         "prompt": {