docs: update some text;

README.md

@@ -46,13 +46,13 @@
     </a>
 </p>
 
-> 📣 TSDB now supports a total of 1️⃣6️⃣8️⃣ time-series datasets ‼️
+> 📣 TSDB now supports a total of 1️⃣6️⃣9️⃣ time-series datasets ‼️
 
 <a href='https://github.com/WenjieDu/PyPOTS'><img src='https://pypots.com/figs/pypots_logos/PyPOTS_logo_FFBG.svg?sanitize=true' width='160' align='left' /></a>
-TSDB is a part of 
+TSDB is a part of
 <a href="https://github.com/WenjieDu/PyPOTS">
 PyPOTS <img align="center" src="https://img.shields.io/github/stars/WenjieDu/PyPOTS?style=social">
-</a> 
+</a>
 (a Python toolbox for data mining on Partially-Observed Time Series), and was separated from PyPOTS for decoupling datasets from learning algorithms.
 
 TSDB is created to help researchers and engineers get rid of data collecting and downloading, and focus back on data processing details. TSDB provides all-in-one-stop convenience for downloading and loading open-source time-series datasets (available datasets listed [below](https://github.com/WenjieDu/TSDB#-list-of-available-datasets)).

tsdb/loading_funcs/electricity_transformer_temperature.py

@@ -25,9 +25,16 @@ def load_ett(local_path):
     Returns
     -------
     data : dict
-        A dictionary contains X:
-            X : pandas.DataFrame
-                The time-series data of Electricity Load Diagrams.
+        A dictionary contains all four sub datasets:
+            ETTm1 : pandas.DataFrame
+                The time-series data of ETTm1
+            ETTm2 : pandas.DataFrame
+                The time-series data of ETTm2
+            ETTh1 : pandas.DataFrame
+                The time-series data of ETTh1
+            ETTh2 : pandas.DataFrame
+                The time-series data of ETTh2
+
     """
     sub_datasets = [
         "ETTm1.csv",