Update long description content type

Signed-off-by: Daniel Lim Wee Soong <weesoong.lim@gmail.com>

Author: daniellimws

setup.py

@@ -53,7 +53,7 @@
     version=__version__,
     description='Generate diagrams from Verilog in Sphinx.',
     long_description=readme,
-    long_description_content_type="text/markdown",
+    long_description_content_type="text/x-rst",
     author="Tim 'mithro' Ansell",
     author_email='me@mith.ro',
     url='https://github.com/SymbiFlow/sphinxcontrib-verilog-diagrams',