fix rows_per_buf definition

Author: skoudoro

nibabel/streamlines/array_sequence.py

@@ -37,7 +37,7 @@ def __init__(self, arr_seq, common_shape, dtype):
         self.common_shape = common_shape
         n_in_row = reduce(mul, common_shape, 1)
         bytes_per_row = n_in_row * dtype.itemsize
-        self.rows_per_buf = bytes_per_row / self.bytes_per_buf
+        self.rows_per_buf = self.bytes_per_buf / bytes_per_row
 
     def update_seq(self, arr_seq):
         arr_seq._offsets = np.array(self.offsets)