Remove reference to the patch_sail_riscv target.

This went away when we moved the sail-riscv submodule to the
CHERIoT-Platform org.

Reported by @gavinhoward

docs/GettingStarted.md

diff --git a/docs/GettingStarted.md b/docs/GettingStarted.md
index 65af676..1aba7fc 100644
--- a/docs/GettingStarted.md
+++ b/docs/GettingStarted.md
@@ -204,7 +204,6 @@
 ```
 $ cd cheriot-sail
 $ eval $(opam env)
-$ make patch_sail_riscv
 $ make csim
 ```