[range.join.with.sentinel] Add missing curly brace (#5530)

Author: hewillk

source/ranges.tex

@@ -6658,7 +6658,8 @@
     template <bool OtherConst>
       requires @\libconcept{sentinel_for}@<sentinel_t<@\exposid{Base}@>, iterator_t<@\exposid{maybe-const}@<OtherConst, V>>>
     friend constexpr bool operator==(const @\exposid{iterator}@<OtherConst>& x, const @\exposid{sentinel}@& y);
-};
+  };
+}
 \end{codeblock}
 
 \begin{itemdecl}