cb0439345cc677ab83a1bbc9a2e5ec442418f515 Fix a few minor errors in the dseries doc.

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 420 | PASS: 418 | FAIL: 0 | XFAIL: 2 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 418 | PASS: 416 | FAIL: 0 | XFAIL: 2 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-master-check-kkxjGePvDf.log'.