help: add a missing blank line to unhide `revlog-compression`

The help was output, but it was elided with "Enabled by default" from the
previous item.

mercurial/help/config.txt

@@ -866,6 +866,7 b' https://www.mercurial-scm.org/wiki/Missi'
     Repositories with this on-disk format require Mercurial version 4.7
 
     Enabled by default.
+
 ``revlog-compression``
     Compression algorithm used by revlog. Supported value are `zlib` and `zstd`.
     The `zlib` engine is the historical default of Mercurial. `zstd` is a newer

tests/test-help.t

@@ -1509,6 +1509,8 b' Separate sections from subsections'
   
       "sparse-revlog"
   
+      "revlog-compression"
+  
       "profiling"
       -----------