Fix new flow casting syntax codegen bug

Reviewed By: gkz

Differential Revision: D95054803

fbshipit-source-id: a66c5bcd5a74f94338a22703c0d214d922aec48c

Author: SamChou19815

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

@@ -1111,7 +1111,7 @@ fn write_concrete_validator_function(
     match typegen_language {
         TypegenLanguage::Flow | TypegenLanguage::JavaScript => {
             let cast_op = if use_new_flow_casting_syntax {
-                " as "
+                ":: as "
             } else {
                 ": "
             };