Fixed pip install instruction (#660)

Added extra dash for extra-index-url

Co-authored-by: samaid <sergey.maidanov@gmail.com>

Author: oleksandr-pavlyk

docs/docfiles/user_guides/QuickStart.rst

@@ -56,7 +56,7 @@ Dpctl can also be istalled from Pypi.
 
 .. code-block:: bash
 
-    python -m pip install --index-url https://pypi.anaconda.org/intel/simple -extra-index-url https://pypi.org/simple dpctl
+    python -m pip install --index-url https://pypi.anaconda.org/intel/simple --extra-index-url https://pypi.org/simple dpctl
 
 .. note::