Update holoscan version (#655)

Update holoscan docker image version to 2.8.0 in the README

Signed-off-by: Bruce Hashemian <3968947+drbeh@users.noreply.github.com>
Co-authored-by: Bruce Hashemian <3968947+drbeh@users.noreply.github.com>

Author: bhashemian

README.md

@@ -89,11 +89,11 @@ Check to verify that the image is created:
 $ docker images
 REPOSITORY      TAG               IMAGE ID       CREATED         SIZE
 ...
-holohub         ngc-v2.1.0-dgpu   17e3aa51f129   13 days ago     13.2GB
+holohub         ngc-v2.8.0-dgpu   17e3aa51f129   13 days ago     13.2GB
 ...
 ```
 
-***Note:*** The development container script ```dev_container``` will by default detect if the system is using an iGPU (integrated GPU) or a dGPU (discrete GPU) and use [NGC's Holoscan SDK container](https://catalog.ngc.nvidia.com/orgs/nvidia/teams/clara-holoscan/containers/holoscan) **`v2.1`** for the [Container build](#container-build-recommended). See [Advanced Container Build Options](/doc/developer.md#advanced-build-options-container) if you would like to use an older version of the SDK as a custom base image.
+***Note:*** The development container script ```dev_container``` will by default detect if the system is using an iGPU (integrated GPU) or a dGPU (discrete GPU) and use [NGC's Holoscan SDK container](https://catalog.ngc.nvidia.com/orgs/nvidia/teams/clara-holoscan/containers/holoscan) **`v2.8`** for the [Container build](#container-build-recommended). See [Advanced Container Build Options](/doc/developer.md#advanced-build-options-container) if you would like to use an older version of the SDK as a custom base image.
 
 See the [Developer Reference document](/doc/developer.md) for additional options.