message: fix a typo in md file
  fix a typo in ml_intro.md, tress -> trees
diff: @@ -25,7 +25,7 @@ components:
  vector responses.
  - Another optional component is the mask of missing measurements. Most algorithms require all the
  components in all the training samples be valid, but some other algorithms, such as decision
  - tress, can handle the cases of missing measurements.
  + trees, can handle the cases of missing measurements.
  - In the case of classification problem user may want to give different weights to different
  classes. This is useful, for example, when:
  - user wants to shift prediction accuracy towards lower false-alarm rate or higher hit-rate.
  