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MINOR: [Docs] Replace "have" with "indicate" in the "Struct validity" section of the docs (#38895)



### Rationale for this change
This documentation [section](https://arrow.apache.org/docs/format/Columnar.html#struct-validity) says:
`...a child array might have a null in its validity bitmap...`
But an array can't have a null in its validity bitmap. It can only indicate a null with a bit equal to 0 in its validity bitmap.

### What changes are included in this PR?
Rewording

### Are these changes tested?
Renders normally

### Are there any user-facing changes?
Changes documentation only.

Authored-by: Stas Stepanov <78556261+stfdxv@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>