update readme for x64 native tools name to be agnostic to Visual Studio versions

Author: mrjbq7

README.md

@@ -101,8 +101,7 @@ and unzip to ``C:\ta-lib``.
 > 3. Download and Install Visual Studio Community (2015 or later)
 >    * Remember to Select ``[Visual C++]`` Feature
 > 4. Build TA-Lib Library
->    * From Windows Start Menu, Start ``[VS2015 x64 Native Tools Command
->      Prompt]``
+>    * From Windows Start Menu, Start ``[x64 Native Tools Command Prompt]``
 >    * Move to ``C:\ta-lib\c\make\cdr\win32\msvc``
 >    * Build the Library ``nmake``