Rename some documentation/comment references to build_image

build_image was renamed to `cros build-image`.  Update comment strings
and docs referencing it.

BUG=b:287468690
TEST=CQ

Change-Id: I212ecc370952bf1686f523dbc464fc8d173066b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4621770
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
diff --git a/api/controller/image_unittest.py b/api/controller/image_unittest.py
index 8b2c93b..5d45a17 100644
--- a/api/controller/image_unittest.py
+++ b/api/controller/image_unittest.py
@@ -385,7 +385,7 @@
     def _CreateMockBuildResult(
         self, image_types: List[int]
     ) -> Optional[image_service.BuildResult]:
-        """Helper to create Mock build_image results.
+        """Helper to create Mock `cros build-image` results.
 
         Args:
             image_types: A list of image types for which the mock BuildResult