Fix new flow casting syntax codegen bug

Reviewed By: gkz

Differential Revision: D94969166

fbshipit-source-id: 274d7a9aed9ca84ee1491f46706facac7f47732f

Author: SamChou19815

compiler/crates/relay-typegen/src/write.rs

@@ -1005,7 +1005,7 @@ fn write_abstract_validator_function(
     match language {
         TypegenLanguage::Flow | TypegenLanguage::JavaScript => {
             let cast_op = if use_new_flow_casting_syntax {
-                " as "
+                ":: as "
             } else {
                 ": "
             };