Fix help doc for 'enable-background-lookup' obsolete setting

The setting name that was settled on is symbols.auto-download.

Author: jasonmolenda

lldb/source/Core/CoreProperties.td

@@ -8,7 +8,7 @@ let Definition = "modulelist" in {
   def EnableBackgroundLookup: Property<"enable-background-lookup", "Boolean">,
     Global,
     DefaultFalse,
-    Desc<"Alias for backward compatibility: when enabled this is the equivalent to 'symbols.download background'.">;
+    Desc<"Alias for backward compatibility: when enabled this is the equivalent to 'symbols.auto-download background'.">;
   def AutoDownload: Property<"auto-download", "Enum">,
     Global,
     DefaultEnumValue<"eSymbolDownloadOff">,