docs: add note about new executable name in migration guide (#2403)

It is a subtle difference to notice in all the examples that the
executable name has changed from using underscores to hyphens.

It is helpful to the reader to have this explicitly pointed out, the
same way that the other changes have been spelled out.

Author: bseib

migration-guide.md

@@ -34,10 +34,26 @@ and then attempt to use a private IP. In v2, the Proxy now defaults to public
 IP without trying private IP. If you want to use private IP, you must pass
 either the `--private-ip` flag or the query parameter. See the README for details.
 
-In some cases, the v1 behavior may be preferrable. Use the `--auto-ip` flag to
+In some cases, the v1 behavior may be preferable. Use the `--auto-ip` flag to
 mimic v1 behavior. We generally recommend using deterministic IP address selection,
 but recognize in some legacy environments `--auto-ip` may be necessary.
 
+## Executable Name Change
+
+Note that the name of the executable has changed, using hyphens rather than underscores:
+
+```shell
+# v1
+./cloud_sql_proxy
+```
+
+vs
+
+```shell
+# v2
+./cloud-sql-proxy
+```
+
 ## Sample Invocations
 
 ### Listen on TCP socket