Fix inaccurate "As mentioned above" assertion

[teogeos]

At some point the article says "As mentioned above, non-exported (lowercase) fields are ignored by the marshaler.". But there is no such previous mention (nor any later mention).

[vercel[bot]]

@teogeos is attempting to deploy a commit to the Boot dev Team on Vercel.

A member of the Team first needs to authorize it.

[teogeos]

Hello @wagslane ! Anything I need to do to have this PR reviewed?

Taking the freedom to ping you since it's been a while and I didn't find more instructions in the contributing guide.

[skovranek]

Thank you, I'm going use this change but add a source as well.