docs: update test for syntax
docs: update test for syntax

Synchronize with commit 2bd9619f7621 ("pkg/runtest: check arch
requirement early"), which removes TestSysTests.

Signed-off-by: Mickaël Salaün <b6edf9c902cda42a2b91f8be7643dbd9a2bf61f4@linux.microsoft.com>

google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller