doc: smf: fix graphviz warning in state diagram

Modified the smf_hierarchical_initial digraph so that it doesn't use a
`<>` label as that makes recent versions of graphviz unhappy (warning).

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/services/smf/index.rst

@@ -551,7 +551,7 @@ state. The statechart for this test is below.
       STATE_B [shape = box];
       STATE_C [shape = box];
       STATE_D [shape = box];
-      DC[shape=point height=0 width=0 label=<>]
+      DC[shape=point height=0 width=0 label="" style="invis"]
 
       subgraph cluster_root {
          label = "ROOT";