Fix typo: "a asynchronous" -> "an asynchronous"

[tadayosi]

Description

Simple typo fix in the openapi specs:

• "a asynchronous" -> "an asynchronous"
• Insert space between two sentences
• Add period at the end of some sentences

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

n/a

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

[mreso]

LGTM, thank you for contributing!