Fix typo

Author: jpintor52

setup.py

@@ -10,7 +10,7 @@
 if __name__ == "__main__":
     setuptools.setup(
         name="pyportfolioopt",
-        version="1.5.7",
+        version="1.5.6",
         description="Financial portfolio optimization in python",
         long_description=desc,
         long_description_content_type="text/markdown",