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.