Fix typo: fo->for

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -362,7 +362,7 @@ fn token_name_eq(t1: &Token, t2: &Token) -> bool {
 }
 
 // Note: the vectors could be created and dropped within `parse_tt`, but to avoid excess
-// allocations we have a single vector fo each kind that is cleared and reused repeatedly.
+// allocations we have a single vector for each kind that is cleared and reused repeatedly.
 pub struct TtParser {
     macro_name: Ident,