Fix main branch iOS prerelease constant (#45398)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45398

Accidentally left in a value (that Phabricator then hid) which I was using to test fixed prerelease constants in D59141948... On real releases, this is overwritten by the version stamping process.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D59668218

fbshipit-source-id: 3d04e52db75a5d8a53cf47d3a2f88b643030d94e

packages/react-native/React/Base/RCTVersion.m

@@ -24,7 +24,7 @@ NSDictionary* RCTGetReactNativeVersion(void)
                   RCTVersionMajor: @(1000),
                   RCTVersionMinor: @(0),
                   RCTVersionPatch: @(0),
-                  RCTVersionPrerelease: @"alpha.0",
+                  RCTVersionPrerelease: [NSNull null],
                   };
   });
   return __rnVersion;