added example code to React Native for setDist (#1670)

Customer wrote into Support about confusion over setDist. This PR gives a code example of how to use setDist.

Author: MimiDumpling

src/collections/_documentation/platforms/react-native/sourcemaps.md

@@ -44,3 +44,7 @@ Sentry.setRelease(RELEASE_NAME)
 `DISTRIBUTION_NAME`:
 
 : This is the version code or build ID depending on your platform. So for instance just set this to whatever is set in your _Info.plist_ or what your Gradle setup generates (For example, `52`).
+
+```js
+Sentry.setDist(DISTRIBUTION_NAME);
+```