[GH] Fix -instance doc in cmd-line-flags.md

This is an imported pull request from #66.

Resolves #66

Test: n/a
Bug: n/a

Github-Pr-Head-Sha: 0c50e0e
GitOrigin-RevId: 7feb6d8add3ffbd7dcd2bf9250b1d02785e2b155
Change-Id: I26421c10993d2cc2c5e916247323396ff8ef41d3

docs/cmd-line-flags.md

@@ -15,9 +15,8 @@ does not need to be set.
 
 **`-instance (string)`**
 
-The instance to target when calling the remote execution service via gRPC. For
-Remote Build Execution, the format is
-`projects/_project-id_/instances/_instance-name_`.
+The instance ID to target when calling remote execution via gRPC
+(e.g., projects/$PROJECT/instances/default_instance for Google RBE).
 
 **`-use_application_default_credentials (bool) (from remote-apis-sdks)`**