rename nullable to parse_opt_nullable

Co-authored-by: Andrew Lamb <[email protected]>

Author: dqkqd

arrow-schema/src/datatype_parse.rs

@@ -449,7 +449,7 @@ impl<'a> Parser<'a> {
     }
 
     /// return and consume if the next token is `Token::Nullable`
-    fn nullable(&mut self) -> bool {
+    fn parse_opt_nullable(&mut self) -> bool {
         self.tokenizer
             .next_if(|next| matches!(next, Ok(Token::Nullable)))
             .is_some()