Update clicommand/agent_start.go

Co-authored-by: Josh Deprez <jd@buildkite.com>

clicommand/agent_start.go

@@ -658,7 +658,7 @@ var AgentStartCommand = cli.Command{
 		},
 		cli.StringFlag{
 			Name:   "signing-jwks-file",
-			Usage:  `Path to a file containing a signing key. Passing this flag enables pipeline signing for all pipelines uploaded by this agent. For hmac-sha256, the raw file content is used as the shared key. When using Docker containers to upload pipeline steps dynamically, use environment variable propagation (for example, "docker run -e MYVAR") to allow all steps within the pipeline to be signed.`,
+			Usage:  `Path to a file containing a signing key. Passing this flag enables pipeline signing for all pipelines uploaded by this agent. For hmac-sha256, the raw file content is used as the shared key. When using Docker containers to upload pipeline steps dynamically, use environment variable propagation (for example, "docker run -e BUILDKITE_AGENT_JWKS_FILE") to allow all steps within the pipeline to be signed.`,
 			EnvVar: "BUILDKITE_AGENT_SIGNING_JWKS_FILE",
 		},
 		cli.StringFlag{