[Format] Permit unsigned dictionary indices in Columnar.rst

[asfimport]

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Implement support for unsigned dictionary indices (is a parent of)
• [Java] Support unsigned dictionary indices (is a parent of)
• [Go] Support unsigned dictionary indices (is a parent of)
• [Rust] Support unsigned dictionary indices (is a parent of)

PRs and other links:

• GitHub Pull Request #7567

_{Note: This issue was originally created as ARROW-9259. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7567
#7567