DOC fix a typo in the documentation (#706)

Author: akanz1

imblearn/__init__.py

@@ -20,12 +20,12 @@
     Module which provides metrics to quantified the classification performance
     with imbalanced dataset.
 over_sampling
-    Module which provides methods to under-sample a dataset.
+    Module which provides methods to over-sample a dataset.
 tensorflow
     Module which provides custom generator, layers for deep learning using
     tensorflow.
 under-sampling
-    Module which provides methods to over-sample a dataset.
+    Module which provides methods to under-sample a dataset.
 utils
     Module including various utilities.
 pipeline