fix(spanner): update the API

#### spanner:v1

The following keys were changed:
- schemas.CommitStats.properties.mutationCount.description

discovery/spanner-v1.json

@@ -2037,7 +2037,7 @@
       }
     }
   },
-  "revision": "20210527",
+  "revision": "20210517",
   "rootUrl": "https://spanner.googleapis.com/",
   "schemas": {
     "Backup": {
@@ -2269,7 +2269,7 @@
       "id": "CommitStats",
       "properties": {
         "mutationCount": {
-          "description": "The total number of mutations for the transaction. Knowing the `mutation_count` value can help you maximize the number of mutations in a transaction and minimize the number of API round trips. You can also monitor this value to prevent transactions from exceeding the system [limit](https://cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data). If the number of mutations exceeds the limit, the server returns [INVALID_ARGUMENT](https://cloud.google.com/spanner/docs/reference/rest/v1/Code#ENUM_VALUES.INVALID_ARGUMENT).",
+          "description": "The total number of mutations for the transaction. Knowing the `mutation_count` value can help you maximize the number of mutations in a transaction and minimize the number of API round trips. You can also monitor this value to prevent transactions from exceeding the system [limit](http://cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data). If the number of mutations exceeds the limit, the server returns [INVALID_ARGUMENT](http://cloud.google.com/spanner/docs/reference/rest/v1/Code#ENUM_VALUES.INVALID_ARGUMENT).",
           "format": "int64",
           "type": "string"
         }

src/apis/spanner/v1.ts

@@ -301,7 +301,7 @@ export namespace spanner_v1 {
    */
   export interface Schema$CommitStats {
     /**
-     * The total number of mutations for the transaction. Knowing the `mutation_count` value can help you maximize the number of mutations in a transaction and minimize the number of API round trips. You can also monitor this value to prevent transactions from exceeding the system [limit](https://cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data). If the number of mutations exceeds the limit, the server returns [INVALID_ARGUMENT](https://cloud.google.com/spanner/docs/reference/rest/v1/Code#ENUM_VALUES.INVALID_ARGUMENT).
+     * The total number of mutations for the transaction. Knowing the `mutation_count` value can help you maximize the number of mutations in a transaction and minimize the number of API round trips. You can also monitor this value to prevent transactions from exceeding the system [limit](http://cloud.google.com/spanner/quotas#limits_for_creating_reading_updating_and_deleting_data). If the number of mutations exceeds the limit, the server returns [INVALID_ARGUMENT](http://cloud.google.com/spanner/docs/reference/rest/v1/Code#ENUM_VALUES.INVALID_ARGUMENT).
      */
     mutationCount?: string | null;
   }