help: add missing `.` in config reference

The `hg help` command is still confused at the number of dots, but at least the
reference is correct.

Differential Revision: https://phab.mercurial-scm.org/D12159

mercurial/helptext/config.txt

@@ -970,8 +970,8 b' https://www.mercurial-scm.org/wiki/Missi'
 
     Beware that existing shares will not be upgraded/downgraded, and by
     default, Mercurial will refuse to interact with them until the mismatch
-    is resolved. See :hg:`help config share.safe-mismatch.source-safe` and
-    :hg:`help config share.safe-mismatch.source-not-safe` for details.
+    is resolved. See :hg:`help config.share.safe-mismatch.source-safe` and
+    :hg:`help config.share.safe-mismatch.source-not-safe` for details.
 
     Introduced in Mercurial 5.7.
 
@@ -2046,7 +2046,7 b' Alias definitions for revsets. See :hg:`'
     tries to upgrade the share; if it fails, continue by
     respecting the share source setting
 
-    Check :hg:`help config format.use-share-safe` for details about the
+    Check :hg:`help config.format.use-share-safe` for details about the
     share-safe feature.
 
 ``safe-mismatch.source-safe.warn``
@@ -2072,7 +2072,7 b' Alias definitions for revsets. See :hg:`'
     tries to downgrade the share to not use share-safe;
     if it fails, continue by respecting the shared source setting
 
-    Check :hg:`help config format.use-share-safe` for details about the
+    Check :hg:`help config.format.use-share-safe` for details about the
     share-safe feature.
 
 ``safe-mismatch.source-not-safe.warn``
@@ -2117,7 +2117,7 b' category impact performance and reposito'
     Default to True.
 
     For details on the "persistent-nodemap" feature, see:
-    :hg:`help config format.use-persistent-nodemap`.
+    :hg:`help config.format.use-persistent-nodemap`.
 
 ``revlog.persistent-nodemap.slow-path``
     Control the behavior of Merucrial when using a repository with "persistent"
@@ -2129,7 +2129,7 b' category impact performance and reposito'
     ``abort``: Prevent access to such repositories. (This is the default)
 
     For details on the "persistent-nodemap" feature, see:
-    :hg:`help config format.use-persistent-nodemap`.
+    :hg:`help config.format.use-persistent-nodemap`.
 
 ``revlog.reuse-external-delta-parent``
     Control the order in which delta parents are considered when adding new