Update line continuations documentation

[laniakea64]

Not sure when this changed, but this is valid now -

foo:
	echo {{ \
 "test" \
 }}

This is still invalid -

foo2:
	echo {{ \
"test" \
 }}

error: Expected backtick, identifier, '(', '/', or string, but found dedent
 ——▶ justfile:3:1
  │
3 │ "test" \
  │ ^

[casey]

LGTM!