Update src/librustc_lexer/src/lib.rs

Co-Authored-By: Ralf Jung <post@ralfj.de>

src/librustc_lexer/src/lib.rs

@@ -1,5 +1,5 @@
 // We want to be able to build this crate with a stable compiler, so feature
-// flags should optional.
+// flags should be optional.
 #![cfg_attr(not(feature = "unicode-xid"), feature(unicode_internals))]
 
 mod cursor;