[doc] Prefer relative links in Markdown

Relative links make it easier to relocate files, prefer using them where
possible.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

hw/ip/otbn/doc/isa.md

diff --git a/hw/ip/otbn/doc/isa.md b/hw/ip/otbn/doc/isa.md
index 9d18d1d..0c267e2 100644
--- a/hw/ip/otbn/doc/isa.md
+++ b/hw/ip/otbn/doc/isa.md
@@ -3,7 +3,7 @@
 ---
 
 This document describes the instruction set for OTBN.
-For more details about the processor itself, see the [OTBN Technical Specification]({{< relref "hw/ip/otbn/doc" >}}).
+For more details about the processor itself, see the [OTBN Technical Specification]({{< relref "." >}}).
 In particular, this document assumes knowledge of the *Processor State* section from that guide.
 
 The instruction set is split into *base* and *big number* subsets.