Use both variables in cli's "base url conflict" error message

`appBaseUrl` was specified twice - which _is_ fine, because the two
values were already proven equal - but it took me until writing this
commit message to realize that it isn't a bug.

So, I'd advocate for this change as "ease of understanding" improvement
:)

Signed-off-by: Mitchell Hentges <mhentges@spotify.com>

.changeset/forty-mangos-fail.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Slightly improve readability of "base URL conflict" error handling code

packages/cli/src/commands/start/startFrontend.ts

@@ -80,7 +80,7 @@ export async function startFrontend(options: StartAppOptions) {
         `⚠️   Conflict between app baseUrl and backend baseUrl:
 
     app.baseUrl:     ${appBaseUrl}
-    backend.baseUrl: ${appBaseUrl}
+    backend.baseUrl: ${backendBaseUrl}
 
     Must have unique hostname and/or ports.