[sw, dif_plic] Substitute '@p' for backticks '`' in comments

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/sw/device/lib/dif/dif_plic.h b/sw/device/lib/dif/dif_plic.h
index 9db636a..2c7f24f 100644
--- a/sw/device/lib/dif/dif_plic.h
+++ b/sw/device/lib/dif/dif_plic.h
@@ -81,7 +81,7 @@
  * Initialises an instance of PLIC.
  *
  * Information that must be retained, and is required to program PLIC, shall
- * be stored in @p plic.
+ * be stored in `plic`.
  *
  * @param base_addr Base address of an instance of the PLIC IP block.
  * @param plic PLIC state data.