doc/developing.md: fixed link to '../dev-packages/private-ext-scripts…

…/README.md' (#13122)

Signed-off-by: Christian Schneider <christian.schneider@typefox.io>

doc/Developing.md

@@ -165,7 +165,7 @@ Theia repository has multiple folders:
  - `packages` folder contains runtime packages, as the core package and extensions to it
  - `dev-packages` folder contains devtime packages
     - [@theia/cli](../dev-packages/cli/README.md) is a command line tool to manage Theia applications
-    - [@theia/ext-scripts](../dev-packages/ext-scripts/README.md) is a command line tool to share scripts between Theia runtime packages
+    - [@theia/ext-scripts](../dev-packages/private-ext-scripts/README.md) is a command line tool to share scripts between Theia runtime packages
  - `examples` folder contains example applications, both Electron-based and browser-based
  - `doc` folder provides documentation about how Theia works
  - `scripts` folder contains JavaScript scripts used by npm scripts when