doc: fix syntax in hns3 guide

[ upstream commit 258e6e1b370c1b3f5e50877b8539511fa248a63a ]

'::' doesn't provide pre-formatted text without an empty line after it,
so fixes it.

Fixes: cdf6a5f ("doc: add runtime option examples to hns3 guide")

Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>

doc/guides/nics/hns3.rst

@@ -81,6 +81,7 @@ Runtime Config Options
   ``common``.
 
   For example::
+
   -a 0000:7d:00.0,rx_func_hint=simple
 
 - ``tx_func_hint`` (default ``none``)
@@ -101,6 +102,7 @@ Runtime Config Options
   ``common``.
 
   For example::
+
   -a 0000:7d:00.0,tx_func_hint=common
 
 - ``dev_caps_mask`` (default ``0``)
@@ -113,6 +115,7 @@ Runtime Config Options
   Its main purpose is to debug and avoid problems.
 
   For example::
+
   -a 0000:7d:00.0,dev_caps_mask=0xF
 
 - ``mbx_time_limit_ms`` (default ``500``)