flags: rename list-flags -> list_flags

Apparently python module names cannot contain -, so naming a script with
a dash prevents importing it as a module. TIL!

TBR because I'm about to mail out some instructions referencing this
file, so it should have the correct name before I do that.

TBR=thakis@chromium.org

Bug: None
Change-Id: I706f5bc25fad29a0a822b30a654893a75e8f45e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760870Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688104}

docs/flag_expiry.md

@@ -30,7 +30,7 @@ developer will not remove it earlier than this process specifies.
 After each milestone's branch point:
 
 1. The flags team chooses a set of flags to begin expiring, from the list
-   produced by `tools/flags/list-flags.py --expired-by $MSTONE`. In the steady
+   produced by `tools/flags/list_flags.py --expired-by $MSTONE`. In the steady
    state, when there is not a big backlog of flags to remove, this set will be
    the entire list of flags that are `expired-by $MSTONE`.
 2. The flags team hides the flags in this set by default from `chrome://flags`,