Fix lexer edition.

Author: ehuss

src/tokens.md

@@ -661,7 +661,7 @@ them are referred to as "token trees" in [macros].  The three types of brackets
 
 ## Reserved prefixes
 
-> **<sup>Lexer 2018+</sup>**\
+> **<sup>Lexer 2021+</sup>**\
 > RESERVED_TOKEN_DOUBLE_QUOTE : ( IDENTIFIER_OR_KEYWORD <sub>_Except `b` or `r` or `br`_</sub> | `_` ) `"`\
 > RESERVED_TOKEN_SINGLE_QUOTE : ( IDENTIFIER_OR_KEYWORD <sub>_Except `b`_</sub> | `_` ) `'`\
 > RESERVED_TOKEN_POUND : ( IDENTIFIER_OR_KEYWORD <sub>_Except `r` or `br`_</sub> | `_` ) `#`