Doc: Fix example manifest file paths

Fix 'examplesinstallpath' to match 'exampledirs' in .qdocconf. This
ensures the generated project paths in manifest files are valid and
the examples are listed in Qt Creator Welcome mode.

Change-Id: I6aaf91e063ff346cced1c39c7411c16b211408e8
Reviewed-by: Caroline Chao <caroline.chao@digia.com>