Fix typo.

Doc/library/codecs.rst

@@ -816,7 +816,7 @@ Without external information it's impossible to reliably determine which
 encoding was used for encoding a Unicode string. Each charmap encoding can
 decode any random byte sequence. However that's not possible with UTF-8, as
 UTF-8 byte sequences have a structure that doesn't allow arbitrary byte
-sequence. To increase the reliability with which a UTF-8 encoding can be
+sequences. To increase the reliability with which a UTF-8 encoding can be
 detected, Microsoft invented a variant of UTF-8 (that Python 2.5 calls
 ``"utf-8-sig"``) for its Notepad program: Before any of the Unicode characters
 is written to the file, a UTF-8 encoded BOM (which looks like this as a byte