WithMany doc: fix the sample expression

[yecril71pl]

A simple non-code change.
blog => blog.Posts
Fixes #26983

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[ajcvickers]

Text should also be updated to indicate that this method is expecting a collection navigation. For example:

    ///     A lambda expression representing the collection navigation property on the other end of this
    ///     relationship (<c>blog => blog.Posts</c>). If no property is specified, the relationship will be
    ///     configured without a navigation property on the other end of the relationship.

[ajcvickers]

Thanks!