lexer: fixed typo

src/knot/conf/cf-lex.l

@@ -169,7 +169,7 @@ critical { lval.t = yytext; lval.i = LOG_UPTO(LOG_CRIT); return LOG_LEVEL; }
 all {
 	lval.t = yytext;
 	lval.i = LOG_UPTO(LOG_INFO);
-	fprintf(stderr, "Warning: log severity 'all'' is no longer "
+	fprintf(stderr, "Warning: log severity 'all' is no longer "
 	                "supported. Using 'info' instead.\n");
 	return LOG_LEVEL;
 }