Corrections to Grammar Errors and Typos in Documentation (#1017)

* Fixed grammar errors and typos.

* Update CHANGELOG.md to mention documentation typo corrections.

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Co-authored-by: Stéphane Bruckert <stephane.bruckert@gmail.com>

examples/app.py

@@ -11,7 +11,7 @@ Prerequisites
     OPTIONAL
     // in development environment for debug output
     export FLASK_ENV=development
-    // so that you can invoke the app outside of the file's directory include
+    // so that you can invoke the app outside the file's directory include
     export FLASK_APP=/path/to/spotipy/examples/app.py
 
     // on Windows, use `SET` instead of `export`