docs: Fixing requirements.txt

 * Install sphinx-verilog-domain.
 * Update for the sphinxcontrib-verilog-diagrams name change.
 * Use the SymbiFlow version of Symbolator.

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

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
index d63f78d..37e7980 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -4,11 +4,13 @@
 sphinx
 sphinx-autobuild
 
+# Verilog domain
+sphinx-verilog-domain
 # Verilog diagrams using Yosys + netlistsvg
-git+https://github.com/SymbiFlow/sphinxcontrib-verilog-diagrams.git#egg=sphinxcontrib-verilog-diagrams
+sphinxcontrib-hdl-diagrams
 
 # Module diagrams
-symbolator
+git+https://github.com/SymbiFlow/symbolator.git#egg=symbolator
 
 # pycairo
 # vext.gi