Metamorph: fix description delimiter to account for XML sanitization

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12633.

To test, verify that the dataset in ```data_repo/from_skyking/metamorph/sebastien-besson/A._WTChlor.2.nd_[Stage1__Position52_]/``` has some original metadata entries that begin with ```#10;```, as noted in the ticket.  Without this change, there should not be any entries that begin with ```#10;```, and e.g. ```Binning``` and ```Temperature``` should be in the original metadata table.

/cc @sbesson