Add entry in table of contents for LCP to M83 changes.

This was erroneously removed from r761522.

TBR=npm@chromium.org

Change-Id: I8245dc8f175b53b31c8b8d90b22ada8306dc6684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163418Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762080}

docs/speed/metrics_changelog/lcp.md

 # Largest Contentful Paint Changelog
 
+* Chrome 83
+  * Metric definition improvement: [Largest Contentful Paint measurement stops at first input or scroll](2020_05_lcp.md)
+  * Metric definition improvement: [Largest Contentful Paint properly accounts for visual size of background images](2020_05_lcp.md)
 * Chrome 81
   * Metric definition improvement: [Largest Text Paint correctly reported while largest image is loading](2020_04_lcp.md)
 * Chrome 79