[flat.set.defn] Fix indentation (#7059)

Author: Andreas-Krug

source/containers.tex

@@ -17022,7 +17022,7 @@
     template<@\exposconcept{container-compatible-range}@<value_type> R, class Alloc>
       flat_set(from_range_t, R&& rg, const Alloc& a);
     template<@\exposconcept{container-compatible-range}@<value_type> R, class Alloc>
-       flat_set(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
+      flat_set(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
     template<class Alloc>
       flat_set(initializer_list<value_type> il, const Alloc& a);
     template<class Alloc>