Fix sx-manifesto EvalError: close string before italic span

The em span variable was embedded inside an unclosed sx string,
causing the " before "italic" to close the outer string and
leaving italic as an undefined bare symbol.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sxc/pages/__init__.py

@@ -2453,8 +2453,8 @@ def _essay_sx_manifesto() -> str:
         'and finally declared itself the universal language of computation. '
         'Like every revolutionary who becomes a tyrant, '
         'it kept the worst habits of the regime it overthrew: '
-        'weak typing, prototype chains, and the '
-        f'  {em} \"this\")"'
+        'weak typing, prototype chains, and the "'
+        f'  {em} \"this\")'
         '      " keyword.")'
         f'    {p}'
         '      "CSS, the third estate, controls all visual presentation '