[forward.list.overview] Fix indentation (#8234)

Author: Andreas-Krug

source/containers.tex

@@ -7104,9 +7104,9 @@
     constexpr void splice_after(const_iterator position, forward_list& x, const_iterator i);
     constexpr void splice_after(const_iterator position, forward_list&& x, const_iterator i);
     constexpr void splice_after(const_iterator position, forward_list& x,
-                      const_iterator first, const_iterator last);
+                                const_iterator first, const_iterator last);
     constexpr void splice_after(const_iterator position, forward_list&& x,
-                      const_iterator first, const_iterator last);
+                                const_iterator first, const_iterator last);
 
     constexpr size_type remove(const T& value);
     template<class Predicate> constexpr size_type remove_if(Predicate pred);