ApproxTimeEvolution.compute_decomposition docstring has example

[isaacdevlugt]

Context:

The docstring of compute_decomposition for ApproxTimeEvolution is slightly misleading with the coeffs_and_time argument.

Description of the Change:

Adds an example to the docstring to clear things up.

Related GitHub Issues:

https://discuss.pennylane.ai/t/approxtimeevo-decomposition-bug/3178/2

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #4354 (df9c8eb) into master (d9382ef) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4354   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files         351      351           
  Lines       32120    32120           
=======================================
  Hits        32053    32053           
  Misses         67       67           

Impacted Files	Coverage Δ
...ane/templates/subroutines/approx_time_evolution.py	100.00% <ø> (ø)

[timmysilv]

aside from Christina's comments, the docs lgtm! thanks for adding this 🎉

[albi3ro]

Thanks 👍