doc: fix grammar and punctuation in child-process documentation

[Rawal27]

Description

Improve the grammar and wording in the child_process documentation.

Changes

• Correct the grammar and wording in the description of the PATH environment variable lookup.
• Correct the possessive form in “the current processes environment variable PATH” to “the current process's environment variable PATH”.
• Improve the wording of the fork() description to use the singular “its own V8 instance”.
• Add the missing article in “Example of checking for a failed spawn”.
• Add the missing comma in “Alternatively, one can redirect...”.
• Correct the possessive form from “child process'” to “child process's”.

Documentation-level changes only. No functional changes.

[aduh95]

Please stop opening PRs faster they can be reviewed. If you do not stop, you will get blocked from the organisation.

[Rawal27]

@aduh95 Can you clarify this? I mean, these are genuine doc-level changes that I have proposed. Also, a contributor can contribute to as many as 30 pull requests at a time. Closing them without reason seems unfair to me.