docs: correct stdin Taskfile command example

[SergioChan]

Summary

• fix the Reading a Taskfile from stdin guide example that used task -t - <(cat ./Taskfile.yml)
• replace it with task -t - < ./Taskfile.yml, which correctly passes stdin to Task when using -t -
• keep the existing piped example (cat ./Taskfile.yml | task -t -) unchanged

Testing

• verified the docs snippet now contains the corrected stdin redirection command:
  • rg -n "task -t - < ./Taskfile.yml|cat ./Taskfile.yml \| task -t -" website/src/docs/guide.md
• note: task CLI is not installed in this runner, so executable command validation was not run locally

Related

Fixes #2610