cmd: start trace short description with T

This fixes a styling issue:

```
$ flux --help

Command line utility for assembling Kubernetes CD pipelines the GitOps
way.

Usage:
  flux [command]

...

Available Commands:
  ...
  suspend     Suspend resources
  trace       trace an in-cluster object throughout the GitOps delivery
pipeline
  uninstall   Uninstall Flux and its custom resource definitions
...
```

Signed-off-by: Hidde Beydals <hello@hidde.co>