helptext: update first hg version when share-safe will be released

I authored the patch which added the helptext before 5.6 release hoping that my
patches will make it. However they didn't before the release and were pushed
after the release only.

Differential Revision: https://phab.mercurial-scm.org/D9410

mercurial/helptext/internals/requirements.txt

@@ -170,7 +170,7 b' they used to be stored. Some working cop'
 in ``.hg/``.
 Shares read the ``.hg/hgrc`` of the source repository.
 
-Support for this requirement was added in Mercurial 5.6 (released
-November 2020). The requirement will only be present on repositories that have
+Support for this requirement was added in Mercurial 5.7 (released
+February 2021). The requirement will only be present on repositories that have
 opted in to this format (by having ``format.exp-share-safe=true`` set when
 they were created).