Docs: improve grammar in plugin.php file header comment

[SAURABHHAM44]

Description

This PR improves the grammar in the file header comment of src/wp-includes/plugin.php.

Change Made: Removed an awkward comma from the line:

• Before: "and filters and hooking functions, and methods."
• After: "and filters and hooking functions and methods."

Why: The comma placement was grammatically incorrect and made the sentence harder to read. This improves clarity for developers reading the plugin API documentation.

Testing:

• No code changes - documentation comment only
• No functional impact
• File syntax remains valid

Note: A corresponding Trac ticket should be created at https://core.trac.wordpress.org/ for this PR to be considered for merging. Please add the ticket number using: Fixes: #[TICKET_NUMBER]