help: update the default value specified for `profiling.time-track` stable

help: update the default value specified for `profiling.time-track`

I tried conditionalizing this in a `.. container::` block, but that seemed to
add an extra blank line between the main text and the parenthetical.

--- a/mercurial/help/config.txt	Wed Oct 24 22:24:10 2018 -0400
+++ b/mercurial/help/config.txt	Wed Oct 24 22:40:48 2018 -0400
@@ -1715,7 +1715,7 @@
 
 ``time-track``
     Control if the stat profiler track ``cpu`` or ``real`` time.
-    (default: ``cpu``)
+    (default: ``cpu`` on Windows, otherwise ``real``)
 
 ``limit``
     Number of lines to show. Specific to the ``ls`` instrumenting profiler.