[Docs] Fix typos in env vars docs (#1429)

Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

docs/source/env_var.rst

@@ -5,9 +5,9 @@ Backend Options
 ---------------
 * ``DGLBACKEND``:
     * Values: String (default='pytorch')
-    * The backend deep lerarning framework for DGL.
+    * The backend deep learning framework for DGL.
     * Choices:
-        * 'pytorch': use PyTorch as the backend implentation.
+        * 'pytorch': use PyTorch as the backend implementation.
         * 'mxnet': use Apache MXNet as the backend implementation.
 
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