the doc-test for backtrace::SpanTrace::capture uses the attributes feature from tracing

[alexanderkjall]

Running cargo test in tracing-error without this change:

---- src/backtrace.rs - backtrace::SpanTrace::capture (line 79) stdout ----
error[E0433]: failed to resolve: could not find `instrument` in `tracing`
  --> src/backtrace.rs:90:12
   |
14 | #[tracing::instrument]
   |            ^^^^^^^^^^ could not find `instrument` in `tracing`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0433`.
Couldn't compile the test.

failures:
    src/backtrace.rs - backtrace::SpanTrace::capture (line 79)

test result: FAILED. 11 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.50s

Motivation

Discovered when packaging the crate for Debian.

Solution

Pull in the required feature as a dev-dependency.

[alexanderkjall]

I looked at the build failures, and they don't look relevant to this PR