Simplify GN target names for //blimp

blimp/docs/container.md

 # Running the engine in a Docker container
 
 For local development and testing, you can run the engine in a Docker
 container.
 
 The steps are:
 
 1. Bundle the engine and its dependencies.
 
 1. Build a Docker image.
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 ### Update Engine Dependencies
 
 `blimp/engine/engine-manifest.txt` is a list of the engine's runtime
 dependencies. From time to time, this list may need to be updated. Use
 `blimp/tools/generate-engine-manifest.py` to (re)generate the manifest:
 
 ```bash
 ./blimp/tools/generate-engine-manifest.py \
     --build-dir out-linux/Debug \
-    --target //blimp/engine:blimp_engine \
+    --target //blimp/engine \
     --output blimp/engine/engine-manifest.txt
 ```
 
 Be sure to review the generated manifest and remove any false runtime
 dependencies.
 
 ## Build Docker Image
 
 Using the tarfile you can create a Docker image:
 
@@ skipping 30 matching lines @@
 ```bash
 docker run -v $CONFIG_DIR:/engine/data -p 443:25466 blimp_engine
 ```
 You can also pass additional flags:
 
 ```bash
 docker run ... blimp_engine --with-my-flags
 ```
 See the [blimp engine `Dockerfile`](../engine/Dockerfile) to find out what flags
 are passed by default.