Reword "`union`s" to "a `union`"

[stu-h]

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As I was reading about unsafe Rust I came across the line "Access fields of unions".

The generated HTML from Markdown displayed the "s" as a separate letter from the intended word "unions".

This led me to think that there was an 's' union, perhaps some sort of sensitive union that you can only access in unsafe mode.

I'm reworded this line to "Access fields of a union" to make it more clear and display cleanly.