Fix typo in GSLOptimizer solver type name.
Fix typo in GSLOptimizer solver type name.

Fix typo in GSL solver type name. Fixes #304.
roystgnr/queso,roystgnr/queso,briadam/queso,roystgnr/queso,roystgnr/queso,briadam/queso,roystgnr/queso,briadam/queso,briadam/queso,briadam/queso,roystgnr/queso