doc/manual: fix formatting slightly

Signed-off-by: Thierry GUIBERT <thierry.guibert@croix-rouge.fr>
[yann.morin.1998@free.fr: split off the previous patch by Thierry]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/adding-board-support.txt

@@ -49,9 +49,10 @@ Buildroot configuration. Refer to xref:customize[] for more details.
 
 Before submitting patches for new boards it is recommended to test it by
 building it using latest gitlab-CI docker container. To do this use
-utils/docker-run script and inside it issue these commands:
++utils/docker-run+ script and inside it issue these commands:
+
 --------------------
- $ make +<boardname>_defconfig+
+ $ make <boardname>_defconfig
  $ make
 --------------------