Fixed typo in windows workflow.

Author: timohl

.github/workflows/windows.yml

@@ -222,7 +222,7 @@ jobs:
           python -m pip install -U jedi==${{ env.JEDI_VER }} idna==${{ env.IDNA_VER }}
       - name: Install Open3D python requirements for stubgen
         working-directory: ${{ env.SOURCE_DIR }}
-        if: ${{ matrix.WITH_STUBGEN == 'ON' }}
+        if: ${{ env.WITH_STUBGEN == 'ON' }}
         run: |
           $ErrorActionPreference = 'Stop'
           python -m pip install -U -r python/requirements.txt