chore: Fix the cloud bigtable link in readme (#1492)

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Fixes #<issue_number_goes_here> ☕️

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Author: mutianf

.readme-partials.yml

@@ -1,7 +1,7 @@
 custom_content: |
   ## About Cloud Bigtable
 
-  [Cloud Bigtable][cloud-bigtable] is Google's NoSQL Big Data database service. It's
+  [Cloud Bigtable][product-docs] is Google's NoSQL Big Data database service. It's
   the same database that powers many core Google services, including Search, Analytics, Maps, and
   Gmail.
 

README.md

@@ -99,7 +99,7 @@ use this Cloud Bigtable Client Library.
 
 ## About Cloud Bigtable
 
-[Cloud Bigtable][cloud-bigtable] is Google's NoSQL Big Data database service. It's
+[Cloud Bigtable][product-docs] is Google's NoSQL Big Data database service. It's
 the same database that powers many core Google services, including Search, Analytics, Maps, and
 Gmail.