Replace HOST/PORT with URL in example

Now that the `microgrid.initialize()` uses a URL, it makes more sense
to specify a URL too.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

examples/battery_pool.py

@@ -13,8 +13,7 @@
 from frequenz.sdk import microgrid
 from frequenz.sdk.actor import ResamplerConfig
 
-HOST = "microgrid.sandbox.api.frequenz.io"  # it should be the host name.
-PORT = 62060
+MICROGRID_API_URL = "grpc://microgrid.sandbox.api.frequenz.io:62060"
 
 
 async def main() -> None: