[lex.charset] Replace "is required" wording in footnote with simple statement.

Author: tkoeppe

source/lex.tex

@@ -216,10 +216,10 @@
 new-line, plus the following 91 graphical characters:\footnote{The glyphs for
 the members of the basic source character set are intended to
 identify characters from the subset of ISO/IEC 10646 which corresponds to the ASCII
-character set. However, because the mapping from source file characters to the source
+character set. However, the mapping from source file characters to the source
 character set (described in translation phase 1) is specified as
 \impldef{mapping from physical source file characters to basic source character set},
-an implementation is required to document how the basic source characters are
+and therefore implementations must document how the basic source characters are
 represented in source files.}
 \begin{codeblock}
 a b c d e f g h i j k l m n o p q r s t u v w x y z