fix(frontend): build messages

The urls are not https://example.org since https://github.com/hedgedoc/hedgedoc/pull/5598 and this message should have been changed then as well.

Signed-off-by: Philip Molares <philip.molares@udo.edu>

frontend/next.config.js

@@ -31,7 +31,7 @@ if (isMockMode) {
 
 if (isBuildTime) {
   console.warn(`This process runs in build mode. During build time this means:
- - Editor and Renderer base urls are https://example.org
+ - Editor and Renderer base urls are '' (empty string)
  - No frontend config will be fetched
 `)
 }