scroll_bar: Fix incorrectly converted breakpoint.
scroll_bar: Fix incorrectly converted breakpoint.

Commit e941ee4a15a0d5e2514882cce24770a02e02829d (#16680) incorrectly
converted this from 775px to xl-max = 1199px instead of md-max =
767px, causing misplacement of the FRB for browser widths between
these values.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,rht/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,hackerkid/zulip