codecs docs: fix grammar mistake (GH-29462)

(cherry picked from commit ea56845)

Co-authored-by: 180909 <[email protected]>

Author: miss-islington

Doc/library/codecs.rst

@@ -909,7 +909,7 @@ there's the so called BOM ("Byte Order Mark"). This is the Unicode character
 ``U+FEFF``. This character can be prepended to every ``UTF-16`` or ``UTF-32``
 byte sequence. The byte swapped version of this character (``0xFFFE``) is an
 illegal character that may not appear in a Unicode text. So when the
-first character in an ``UTF-16`` or ``UTF-32`` byte sequence
+first character in a ``UTF-16`` or ``UTF-32`` byte sequence
 appears to be a ``U+FFFE`` the bytes have to be swapped on decoding.
 Unfortunately the character ``U+FEFF`` had a second purpose as
 a ``ZERO WIDTH NO-BREAK SPACE``: a character that has no width and doesn't allow