Fixes a typo in expiration.rst

[kane-menicou]

Fixes a small typo in the expiration.rst file

[javiereguiluz]

Kane, thanks for fixing this grammar issue. However, even if the phrase is correct now, I think it's wrong to say that: "you may recall that ..." Why should I recall that? Here's a reword proposal:

Before:

Most of the time, you will use the ``Cache-Control`` header. You may
recall that the ``Cache-Control`` header is used to specify many
different cache directives::

After:

Most of the time, you will use the ``Cache-Control`` header, which
is used to specify many different cache directives::

Would that read better to you? Thanks.

[kane-menicou]

Yeah that makes much more sense, thanks.

[javiereguiluz]

Thanks ... and congrats on your first Symfony Docs contribution!