Making name of s4ext file match the name of the extension

Slicer emits an error when installing this extension.  Error is from the ctk::copyDirRecursively() when an unknown directory. Hypothesis is that the name of s4ext file does not match the name of the extension.  The packaging if the extension is based on the name of the extension. The installation of the extension is based on the name of the s4ext file.

This patch only renames the s4ext file in the index. No other code changes.

Interestingly, the s4ext file here does not match what is generated when you do a build of their extension.  They may need to update the s4ext file in the ExtensionIndex to the latest version of the module.