[Docs] Fix config example for "Removal of missing projects" (#1955)

Fixes #1954

Author: jf

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## master (unreleased)
 
 ### Changes
+* [#1954](https://github.com/bbatsov/projectile/issues/1954): update ELisp for usage.html / "Removal of missing projects"
 * [#1947](https://github.com/bbatsov/projectile/issues/1947): `projectile-project-name` should be marked as safe
 * Set `projectile-auto-discover` to `nil` by default.
 * [#1943](https://github.com/bbatsov/projectile/pull/1943): Consider `projectile-indexing-method` to be safe as a dir-local variable if it is one of the preset values.

doc/modules/ROOT/pages/usage.adoc

@@ -87,7 +87,7 @@ around. (e.g. they were removed or renamed) You can either trigger the command
 
 [source,elisp]
 ----
-(setq projectile-cleanup-known-projects nil)
+(customize-set-variable 'projectile-auto-cleanup-known-projects t)
 ----
 
 TIP: If you're a heavy TRAMP user it's probably not a good idea to auto-discover