Update React-RCTAppDelegate.podspec syntax error

The `USE_HERMES` flag is set not escaped correctly by `React-RCTAppDelegate.podspec`, and it does not apply.

The flag needs to be defined as `USE_HERMES\=1`, but is currently set as `USE_HERMES`.

Hopefully this will fix facebook#38193.

packages/react-native/Libraries/AppDelegate/React-RCTAppDelegate.podspec

@@ -25,7 +25,7 @@ use_hermes = ENV['USE_HERMES'] == nil || ENV['USE_HERMES'] == '1'
 
 new_arch_enabled_flag = (is_new_arch_enabled ? " -DRCT_NEW_ARCH_ENABLED" : "")
 is_fabric_enabled = true #is_new_arch_enabled || ENV["RCT_FABRIC_ENABLED"]
-hermes_flag = (use_hermes ? " -DUSE_HERMES" : "")
+hermes_flag = (use_hermes ? " -DUSE_HERMES=1" : "")
 other_cflags = "$(inherited) " + folly_compiler_flags + new_arch_enabled_flag + hermes_flag
 
 header_search_paths = [