Use GitHub upstream git for Verilator

Verilator has moved to GitHub, use the new upstream for a more reliable
git clone experience.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>
diff --git a/doc/ug/install_instructions/index.md b/doc/ug/install_instructions/index.md
index edb5e11..b9df596 100644
--- a/doc/ug/install_instructions/index.md
+++ b/doc/ug/install_instructions/index.md
@@ -172,7 +172,7 @@
 ```console
 $ export VERILATOR_VERSION={{< tool_version "verilator" >}}
 
-$ git clone http://git.veripool.org/git/verilator
+$ git clone https://github.com/verilator/verilator.git
 $ cd verilator
 $ git checkout v$VERILATOR_VERSION