Fixed a 404 in README.md. (#159)

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### Summary

The readme's "Rally Health" link is a 404.  It appears to attempt to link to a careers page.  I skillfully sniffed out a suitable careers page by scrolling down on the 404 page and clicking "careers".

### Test plan

0. Scroll to the bottom of `README.md`;
0. Middle click on "Rally Health";
0. Bask (optional).

Author: Asday

README.md

@@ -88,7 +88,7 @@ React Native Testing Library is an open source project and will always remain fr
 
 ---
 
-Supported and used by [Rally Health](https://www.rallyhealth.com/company/careers).
+Supported and used by [Rally Health](https://www.rallyhealth.com/careers-home).
 
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