Update completer documentation (#13999)

The docstring for suppression says that `do_not_suppress` should be a
list when it should actually be a set

IPython/core/completer.py

@@ -163,7 +163,7 @@
 the matcher with higher priority will be returned.
 
 Sometimes it is desirable to suppress most but not all other matchers;
-this can be achieved by adding a list of identifiers of matchers which
+this can be achieved by adding a set of identifiers of matchers which
 should not be suppressed to ``MatcherResult`` under ``do_not_suppress`` key.
 
 The suppression behaviour can is user-configurable via