F1 Help Doesn't work for *.pro files

STEPS:
1. Install qt-linux-opensource-5.0.0-x86_64-offline-2012-12-18-389.run on Ubuntu 12.04 64 bit.
2. Run Qt Creator.
3. Open 2dpainting project.
4. Go to 2dpainting.pro file.
5. Place your cursor in HEADERS or SOURCES word.
6. Press F1.

ACTUAL RESULT:
Error loading: qthelp://com.trolltech.qmake/qdoc/qmake-variable-reference.html#headers

EXPECTED RESULT:

• Relevant qmake documentation should be shown.