[iterator.requirements.general] Revert indirectly_writable to "writable" definition (#7471)

This fixes a misapplication of the 2019 Belfast meeting LWG motion 9 (P1878R1), which erroneously replaced the "writable" definition by the `indirectly_writable` concept.

Author: Dani-Hub

source/iterators.tex

@@ -539,7 +539,7 @@
 \defn{value type}
 of the iterator.
 An output iterator \tcode{i} has a non-empty set of types that are
-\libconcept{indirectly_writable} to the iterator;
+\defn{writable} to the iterator;
 for each such type \tcode{T}, the expression \tcode{*i = o}
 is valid where \tcode{o} is a value of type \tcode{T}.
 For every iterator type