RelNotes 2.18: minor fix to entry about dynamically loading completions

It was not "newer versions of bash" but newer versions of
bash-completion that made commit 085e2ee0 (completion: load
completion file for external subcommand, 2018-04-29) both necessary
and possible.

Update the corresponding RelNotes entry accordingly.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.18.0.txt

@@ -104,7 +104,7 @@ UI, Workflows & Features
  * The command line completion mechanism (in contrib/) learned to load
    custom completion file for "git $command" where $command is a
    custom "git-$command" that the end user has on the $PATH when using
-   newer version of bash.
+   newer version of bash-completion.
  * "git send-email" can sometimes offer confirmation dialog "Send this
    email?" with choices 'Yes', 'No', 'Quit', and 'All'.  A new action