polish: fix lexer state comment (graphql#4294)

we must update the lexer line number and the line start position,
because lookahead saves the token within the linked list, and so will
never be called again on this token

we do not change the current token, however, until the lexer is advanced

closes graphql#2764

Author: yaacovCR

src/language/lexer.ts

@@ -78,7 +78,7 @@ export class Lexer implements LexerInterface {
 
   /**
    * Looks ahead and returns the next non-ignored token, but does not change
-   * the state of Lexer.
+   * the current Lexer token.
    */
   lookahead(): Token {
     let token = this.token;