cmd/rng: fix long help text

The number of bytes may only be specified if a device number id provided.

Correct the formatting.

Acked-by: Marek BehĂșn <kabel@kernel.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/cmd/rng.rst b/doc/usage/cmd/rng.rst
index 4a61e33..c071f01 100644
--- a/doc/usage/cmd/rng.rst
+++ b/doc/usage/cmd/rng.rst
@@ -12,14 +12,14 @@
 ::
 
     rng list
-    rng [dev] [n]
+    rng [dev [n]]
 
 rng list
 --------
 
 List all the probed rng devices.
 
-rng [dev] [n]
+rng [dev [n]]
 -------------
 
 The *rng* command reads the random number generator(RNG) device and