Fix minor typo in good_toplevel_metrics.md

Updating "its computed" to "it is computed"

Bug: 363034680
Change-Id: Ied5127e0be61d47a84799365d69f83707b22ee60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5889446
Reviewed-by: Sam Fortiner <samfort@microsoft.com>
Commit-Queue: Ognjen Sobajic <ognjen.sobajic@microsoft.com>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1359706}

docs/speed/good_toplevel_metrics.md

@@ -54,7 +54,7 @@ Metrics which are easy to interpret are often easier to evaluate. For example, F
 
 ## Simple
 
-A metric is simple if the way its computed is easy to understand. There’s a strong correlation between being simple and being interpretable, but there are counter examples, such as FMP being interpretable, but not simple.
+A metric is simple if the way it is computed is easy to understand. There’s a strong correlation between being simple and being interpretable, but there are counter examples, such as FMP being interpretable, but not simple.
 
 A simple metric is less likely to have been overfit during the metric development / evaluation phase, and has other obvious advantages (easier to maintain, often faster to execute, less likely to contain bugs).