Update deploy.rst

Author: yuanqing-wang

docs/deploy.rst

@@ -9,7 +9,7 @@ systems in a few lines::
     import espaloma as esp
 
     # grab pretrained model
-    if not os.exists("espaloma_model.pt"):
+    if not os.path.exists("espaloma_model.pt"):
         os.system("wget http://data.wangyq.net/espaloma_model.pt")
 
     # define or load a molecule of interest via the Open Force Field toolkit