VariableManager: Fix documentation

${var} has been replaced with %{var} since 2.2

Change-Id: Ifecd94fb68b8f7dfb4b4f56736d8c317cd6d41a2
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>