DOC: Change the web page to state 1.15 as the minimum required version of NumPy.

Author: WarrenWeckesser

docweb/index.html

@@ -49,7 +49,7 @@ <h2 id="installation">Installation</h2>
   $ pip install numpy-financial
 </pre>
 
-The package requires NumPy version 1.17 or later.
+The package requires NumPy version 1.15 or later.
 </p>
 
 <h2 id="using">Using <code>numpy_financial</code></h2>