5ttgen msmt: Fix copyright header

Erroneous change introduced in 2bc2089.

python/mrtrix3/commands/5ttgen/msmt.py

@@ -4,7 +4,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
-# Covered Software is provided under this License on an 'as is'
+# Covered Software is provided under this License on an "as is"
 # basis, without warranty of any kind, either expressed, implied, or
 # statutory, including, without limitation, warranties that the
 # Covered Software is free of defects, merchantable, fit for a