docs: remove stale reference to plugins page

[Murderlon]

Closes #5402

[github-actions]

Diff output files

diff --git a/packages/@uppy/core/lib/Uppy.js b/packages/@uppy/core/lib/Uppy.js
index de40894..afd06a8 100644
--- a/packages/@uppy/core/lib/Uppy.js
+++ b/packages/@uppy/core/lib/Uppy.js
@@ -836,7 +836,7 @@ export class Uppy {
     const existsPluginAlready = this.getPlugin(pluginId);
     if (existsPluginAlready) {
       const msg = `Already found a plugin named '${existsPluginAlready.id}'. ` + `Tried to use: '${pluginId}'.\n`
-        + "Uppy plugins must have unique `id` options. See https://uppy.io/docs/plugins/#id.";
+        + "Uppy plugins must have unique `id` options.";
       throw new Error(msg);
     }
     if (Plugin.VERSION) {