fix: Preconditions only supports %s format

gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -536,7 +536,7 @@ public int getPoolSize() {
     public Builder setPoolSize(int poolSize) {
       Preconditions.checkArgument(poolSize > 0, "Pool size must be positive");
       Preconditions.checkArgument(
-          poolSize <= MAX_POOL_SIZE, "Pool size must be less than %d", MAX_POOL_SIZE);
+          poolSize <= MAX_POOL_SIZE, "Pool size must be less than %s", MAX_POOL_SIZE);
       this.poolSize = poolSize;
       return this;
     }