Docs/mm: fix typos and grammar in page_tables.rst

Docs/mm: fix typos and grammar in page_tables.rst

Correct several spelling and grammatical errors in the page tables
documentation. This includes:
- Fixing "a address" to "an address"
- Fixing "pfs" to "pfns"
- Correcting the possessive "Torvald's" to "Torvalds's"
- Fixing "instruction that want" to "instruction that wants"
- Fixing "code path" to "code paths"

Signed-off-by: Min-Hsun Chang <chmh0624@gmail.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260209145603.96664-1-chmh0624@gmail.com>