Update README.md (#209)

Author: JingchaoZhang

README.md

@@ -257,7 +257,7 @@ If a valence of 3 is detected on carbon, the formal charge will be assigned to -
 
 # Plugins
 
-One can follow [a simple example](plugin_example/) to add their own format by creating and installing plugins. It's crirical to add the [Format](dpdata/format.py) class to `entry_points['dpdata.plugins']` in `setup.py`:
+One can follow [a simple example](plugin_example/) to add their own format by creating and installing plugins. It's critical to add the [Format](dpdata/format.py) class to `entry_points['dpdata.plugins']` in `setup.py`:
 ```py
     entry_points={
         'dpdata.plugins': [