ENH: Add a missing white space between words in string literal.

Add a missing white space between words in a string literal split in two
lines.

Author: jhlegarreta

nibabel/streamlines/trk.py

@@ -606,7 +606,7 @@ def _read_header(fileobj):
             # http://trackvis.org/blog/forum/diffusion-toolkit-usage/interpretation-of-track-point-coordinates
             if header[Field.VOXEL_ORDER] == b"":
                 msg = ("Voxel order is not specified, will assume 'LPS' since"
-                       "it is Trackvis software's default.")
+                       " it is Trackvis software's default.")
                 warnings.warn(msg, HeaderWarning)
                 header[Field.VOXEL_ORDER] = b"LPS"