Rephrase the input of compare_apk_size

I thought this was asking to give a number of PRs. So multiple one pr,
two pr...

I hope this will save some other dev some confusion, and they'll
directly understand they need a PR number.

.github/workflows/compare_apk_size.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
     inputs:
       prNumber:
-        description: "Number of PR to calculate sizes for"
+        description: "You want to measure the impact of PR number:"
         required: true
         type: number