doc: drop the `exp-` prefix from the `share-safe` requirement

I can't tell if we should leave the experimental one around for historical
documentation purposes, but I'm not sure if there's value to doing that.

Differential Revision: https://phab.mercurial-scm.org/D9849

mercurial/helptext/internals/requirements.txt

@@ -156,8 +156,8 b' Note that as of 5.5, only installations '
 benefit from a speedup. The other installations will do the necessary work to
 keep the index up to date, but will suffer a slowdown.
 
-exp-sharesafe
-=============
+share-safe
+==========
 
 Represents that the repository can be shared safely. Requirements and config of
 the source repository will be shared.