Minor text changes to /open, /join and /mute slash commands (#8569)

* Update en.json

* Update en.json

* Update en.json

* Fix bad rebasing

* ping jenkins

i18n/en.json

@@ -796,7 +796,7 @@
   },
   {
     "id": "api.command_join.hint",
-    "translation": "[channel-name]"
+    "translation": "~[channel]"
   },
   {
     "id": "api.command_join.list.app_error",
@@ -904,11 +904,11 @@
   },
   {
     "id": "api.command_mute.error",
-    "translation": "Could not find the channel {{.Channel}}."
+    "translation": "Could not find the channel {{.Channel}}. Please use the [channel handle](https://about.mattermost.com/default-channel-handle-documentation) to identify channels."
   },
   {
     "id": "api.command_mute.hint",
-    "translation": "[channel]"
+    "translation": "~[channel]"
   },
   {
     "id": "api.command_mute.name",