[iterator.requirements.general] Revert indirectly_writable to *writ…

[Dani-Hub]

…able* definition

This fixes a misapplication of the 2019 Belfast meeting LWG motion 9 (P1878R1), which erroneously replaced the writable definition by the indirectly_writable concept (See issue #7470).

Fixes #7470

[jwakely]

The paper only said to replace the uses of the writable concept, not all cases of the word "writable"

[jwakely]

The change was part of 333b6eb i.e. #3451

It also changed:

@ -1258,16 +1272,16 @@ [iterator.concept.writable]
 \pnum
 \begin{note}
 The only valid use of an \tcode{operator*} is on the left side of the assignment statement.
-Assignment through the same value of the writable type happens only once.
+Assignment through the same value of the indirectly writable type happens only once.
 \end{note}

but I think this is OK.

[jensmaurer]

Let's make it so.