help: use the correct spelling for `slow-path` in persistent nodemap help stable

help: use the correct spelling for `slow-path` in persistent nodemap help

Differential Revision: https://phab.mercurial-scm.org/D11213

--- a/mercurial/helptext/config.txt	Mon Mar 11 10:59:35 2019 -0700
+++ b/mercurial/helptext/config.txt	Mon Jul 26 21:52:10 2021 +0200
@@ -906,7 +906,7 @@
     with the Rust part  might actually suffer some slowdown. For this reason,
     Such version will by default refuse to access such repositories. That
     behavior can be controlled by configuration. Check
-    :hg:`help config.storage.revlog.persistent-nodemap.slowpath` for details.
+    :hg:`help config.storage.revlog.persistent-nodemap.slow-path` for details.
 
     Repository with this on-disk format require Mercurial version 5.4 or above.