[docs] Update out of date compile step

Upstream-repository: operator-registry
Upstream-commit: 9d1c8fb2b4202c94ff7565065d0d3a564bd126a4

Author: kevinrizza

staging/operator-registry/docs/design/operator-bundle.md

@@ -125,7 +125,7 @@ $ git clone https://github.com/operator-framework/operator-registry
 2. Build `opm` binary using this command:
 
 ```bash
-$ go build ./cmd/opm/
+$ make build
 ```
 
 Now, a binary named `opm` is now built in current directory and ready to be used.