style: fix underlines

Author: d-v-b

docs/v3/core/v3.0.rst

@@ -1650,7 +1650,7 @@ This section is non-normative and presents notes from implementers about cases
 that need to be carefully considered but do not strictly fall into the spec.
 
 Explicit vs implicit groups
---------------------------
+---------------------------
 
 This specification defines both implicit and explicit groups, but implementations may
 create an explicit group for all implicit groups they encounter, in