Update usgscsm_cam_test.rst

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This program is shipped with the USGSCSM library in the ``bin`` directory.

It can be used for performing several operations involving CSM camera

models, such as loading a camera model, whether in the original ISD format,

model state representation, or GXP .sup file exporting the model state, computing

projections from pixels in the camera to the ground and back, and

then verifying that the original pixels are obtained.

It can perform several operations involving CSM camera models, such as:

 - Load a camera model, whether in the original ISD format, model state representation, or GXP .sup file

 - Export the camera as a model state file

 - Compute projections from pixels in the camera to the ground and back, and measure the discepancy with the original pixels.

Example (load a camera model and save the model state)::