Fix path to scisql_index in documentation

In order be get eups-compliant, scisql_index is
installed by default in scisql_prefix/bin.
Documentation has been updated w.r.t. this.
Tests, which are launching documentation extracts,
are now passed successfully.
Merge pull request #7 from fjammes/u/fjammes/DM-1279

Fix path to scisql_index in documentation