[hmac] fix the doc headings for Hugo

Some remnants of mistletoe were in the HMAC doc. Removed

hw/ip/hmac/doc/_index.md

diff --git a/hw/ip/hmac/doc/_index.md b/hw/ip/hmac/doc/_index.md
index 2165827..ef33f30 100644
--- a/hw/ip/hmac/doc/_index.md
+++ b/hw/ip/hmac/doc/_index.md
@@ -241,7 +241,7 @@
 }
 ```
 
-## Trigger HMAC/SHA-256 engine }}
+## Trigger HMAC/SHA-256 engine
 
 The following code shows how to send a message to the HMAC, the proceedure is
 the same whether a full HMAC or SHA-256 calculation only is wanted (choose
@@ -277,7 +277,7 @@
 }
 ```
 
-## Interrupt Handling }}
+## Interrupt Handling
 
 ### FIFO_FULL