Fix a typo: if you requires --> if you require (dotnet#5330)

Author: jeffhandley

xml/System.Text/Encoding.xml

@@ -516,7 +516,7 @@ The following table lists the encodings supported by .NET. It lists each encodin
 
 - The ASCII encoding is usually appropriate for protocols that require ASCII.
 
-- If you requires 8-bit encoding (which is sometimes incorrectly referred to as "ASCII"), the UTF-8 encoding is recommended over the ASCII encoding. For the characters 0-7F, the results are identical, but use of UTF-8 avoids data loss by allowing representation of all Unicode characters that are representable. Note that the ASCII encoding has an 8th bit ambiguity that can allow malicious use, but the UTF-8 encoding removes ambiguity about the 8th bit.
+- If you require 8-bit encoding (which is sometimes incorrectly referred to as "ASCII"), the UTF-8 encoding is recommended over the ASCII encoding. For the characters 0-7F, the results are identical, but use of UTF-8 avoids data loss by allowing representation of all Unicode characters that are representable. Note that the ASCII encoding has an 8th bit ambiguity that can allow malicious use, but the UTF-8 encoding removes ambiguity about the 8th bit.
 
 - Prior to .NET Framework version 2.0, .NET Framework allowed spoofing by ignoring the 8th bit. Beginning with .NET Framework 2.0, non-ASCII code points fall back during decoding.