Fix typos in docs (#159)

Found some minor typos in docs after review.

docs/source/background/datatypes.rst

@@ -94,7 +94,7 @@ int vector list
 ~~~~~~~~~~~~~~~~~~
 :NRRD Syntax: (<i>,<i>,...,<i>) (<i>,<i>,...,<i>) ... (<i>,<i>,...,<i>)
 :NRRD Example: (1,0,0) (0,1,0) none (0,0,1)
-:Python Datatype: (M,N) :class:`list` of (N,) :class:`numpy.ndarray` of :class:`int`
+:Python Datatype: :class:`list` of (N,) :class:`numpy.ndarray` of :class:`int`
 :Python Example: [np.array([1, 0, 0]), np.array([0, 1, 0]), None, np.array([0, 0, 1])]
 
 This datatype is similar to `int matrix`_ except instead of returning a (M,N) :class:`numpy.ndarray`, it returns a list of (N,) :class:`numpy.ndarray`. Each row is optional and designated by :code:`none` in the NRRD specification and represented as :obj:`None` in this library.
@@ -103,7 +103,7 @@ double vector list
 ~~~~~~~~~~~~~~~~~~
 :NRRD Syntax: (<d>,<d>,...,<d>) (<d>,<d>,...,<d>) ... (<d>,<d>,...,<d>)
 :NRRD Example: (2.54, 1.3, 0.0) (3.14, 0.3, 3.3) none (0.05, -12.3, -3.3)
-:Python Datatype: (M,N) :class:`list` of (N,) :class:`numpy.ndarray` of :class:`float`
+:Python Datatype: :class:`list` of (N,) :class:`numpy.ndarray` of :class:`float`
 :Python Example: [np.array([2.54, 1.3, 0.0]), np.array([3.14, 0.3, 3.3]), None, np.array([0.0, -12.3, -3.3])]
 
 This datatype is similar to `double matrix`_ except instead of returning a (M,N) :class:`numpy.ndarray`, it returns a list of (N,) :class:`numpy.ndarray`. Each row is optional and designated by :code:`none` in the NRRD specification and represented as :obj:`None` in this library.