netlink: specs: rt-link: replace underscores with dashes in names

We're trying to add a strict regexp for the name format in the spec.
Underscores will not be allowed, dashes should be used instead.
This makes no difference to C (codegen, if used, replaces special
chars in names) but it gives more uniform naming in Python.

Fixes: b2f63d904e ("doc/netlink: Add spec for rt link messages")
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250624211002.3475021-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt-link.yaml

@@ -603,7 +603,7 @@ definitions:
         name: optmask
         type: u32
   -
-    name: if_stats_msg
+    name: if-stats-msg
     type: struct
     members:
       -
@@ -2486,7 +2486,7 @@ operations:
       name: getstats
       doc: Get / dump link stats.
       attribute-set: stats-attrs
-      fixed-header: if_stats_msg
+      fixed-header: if-stats-msg
       do:
         request:
           value: 94