Fix typo in versionadded annotation

Author: vkarak

reframe/core/launchers/__init__.py

@@ -54,7 +54,7 @@ class JobLauncher(metaclass=_JobLauncherMeta):
  #: :type: :class:`List[str]`
  #: :default: ``[]``
  #:
- #: :versionadded:: 4.6.0
+ #: .. versionadded:: 4.6.0
  modifier_options = variable(typ.List[str], value=[])
 
  def __init__(self):