fix formatting error in manual

Author: mfeurer

doc/manual.rst

@@ -68,7 +68,7 @@ For a full list please have a look at the source code (in `autosklearn/pipeline/
   * `Preprocessors <https://github.com/automl/auto-sklearn/tree/master/autosklearn/pipeline/components/feature_preprocessing>`_
 
 Turning off preprocessing
-~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Preprocessing in *auto-sklearn* is divided into data preprocessing and
 feature preprocessing. Data preprocessing includes One-Hot encoding of