Make assist_satellite action descriptions consistent (#136955)

- use third-person singular for descriptive language, following HA standards
- use "a satellite" in both descriptions to match
- use sentence-casing for "Start conversation" action name

Author: NoRi2909

homeassistant/components/assist_satellite/strings.json

@@ -14,7 +14,7 @@
   "services": {
     "announce": {
       "name": "Announce",
-      "description": "Let the satellite announce a message.",
+      "description": "Lets a satellite announce a message.",
       "fields": {
         "message": {
           "name": "Message",
@@ -27,8 +27,8 @@
       }
     },
     "start_conversation": {
-      "name": "Start Conversation",
-      "description": "Start a conversation from a satellite.",
+      "name": "Start conversation",
+      "description": "Starts a conversation from a satellite.",
       "fields": {
         "start_message": {
           "name": "Message",