Rename proposer_exists (#11844)

beacon-chain/rpc/prysm/v1alpha1/validator/BUILD.bazel

@@ -18,7 +18,7 @@ go_library(
         "proposer_empty_block.go",
         "proposer_eth1data.go",
         "proposer_execution_payload.go",
-        "proposer_exists.go",
+        "proposer_exits.go",
         "proposer_phase0.go",
         "proposer_slashings.go",
         "proposer_sync_aggregate.go",
@@ -180,7 +180,7 @@ go_test(
         "proposer_deposits_test.go",
         "proposer_empty_block_test.go",
         "proposer_execution_payload_test.go",
-        "proposer_exists_test.go",
+        "proposer_exits_test.go",
         "proposer_slashings_test.go",
         "proposer_sync_aggregate_test.go",
         "proposer_test.go",