message: Fix minor typo in Programmer's Guide docs
diff: @@ -147,7 +147,7 @@ Passing a single number, however, returns a subvector of a matrix, as follows:
  ```python
  -my_row_vetor = my_matrix[2]
  +my_row_vector = my_matrix[2]
  my_column_vector = my_matrix[:, 3]
  ```
  