docs: Fix name in the extensions list.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

docs/conf.py

diff --git a/docs/conf.py b/docs/conf.py
index 31fb98f..ecc4c9d 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -64,7 +64,7 @@
     'sphinx.ext.mathjax',
     'sphinx.ext.napoleon',
     'sphinx.ext.todo',
-    'sphinxcontrib_verilog_diagrams',
+    'sphinxcontrib_hdl_diagrams',
 ]
 
 # Add any paths that contain templates here, relative to this directory.