Minor update of documentation (spell-check)

[florence-bockting]

Summary

Running devtools::spell_check() revealed some minor typos and en-US/en-GB inconsistencies.
This PR fixes the detected inconsistencies in the documentation assuming en-US as default.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[paul-buerkner]

Thank you!

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if e41819d is merged into master:

• ✔️as_draws_array: 130ms -> 130ms [-0.78%, +0.77%]
• ✔️as_draws_df: 68.9ms -> 68.7ms [-1.47%, +0.93%]
• ✔️as_draws_list: 156ms -> 156ms [-0.47%, +1.38%]
• ✔️as_draws_matrix: 26.1ms -> 26.1ms [-1.06%, +1.15%]
• ✔️as_draws_rvars: 121ms -> 119ms [-3.8%, +0.9%]
• ✔️summarise_draws_100_variables: 735ms -> 734ms [-0.65%, +0.59%]
• ✔️summarise_draws_10_variables: 86.5ms -> 87.3ms [-0.37%, +2.03%]
  Further explanation regarding interpretation and methodology can be found in the documentation.