compose box: Rename 'message' to 'completedText' in 'handleMessageAut…

…ocomplete'.

Renames the 'message' parameter in the 'handleMessageAutocomplete'
method in 'ComposeBox' to 'completedText' for better clarity, so
that we don't think of the 'Message' type.

src/compose/ComposeBox.js

@@ -191,8 +191,12 @@ class ComposeBox extends PureComponent<Props, State> {
   };
 
   // See JSDoc on 'onAutocomplete' in 'AutocompleteView.js'.
-  handleMessageAutocomplete = (message: string, completion: string, lastWordPrefix: string) => {
-    this.setMessageInputValue(message);
+  handleMessageAutocomplete = (
+    completedText: string,
+    completion: string,
+    lastWordPrefix: string,
+  ) => {
+    this.setMessageInputValue(completedText);
 
     if (lastWordPrefix === '@') {
       if (this.mentionWarnings.current) {