docs(manual): standardize ruled-table rule style on a solid dash run

Every ruled table in the SSOT used one solid rule under the header
except the "Fish Universal Variables" table, which used the other
(per-column, RST-style) convention build-manual.py just learned to
also accept. Two conventions for the same thing with no reason for
the split, so convert the outlier to match the other six.

The parser keeps supporting both — the per-column style stays a
useful fallback for anyone still authoring that way — this just
makes our own SSOT consistent.

docs/fish-config.md

@@ -2528,7 +2528,7 @@ switch: setting __fish_config_opinionated=0 disables all unset categories,
 but a category with an explicit truthy value remains enabled regardless.
 
     Variable                        Disables
-    ------------------------------  ------------------------------------
+    ────────────────────────────────────────
     __fish_config_op_aliases        Command shadows and flag injection:
                                     ls->eza, cat->bat, cd->zoxide,
                                     rm->trash, less->ov, top->btop,