DOC: Sync the license file with the current Slicer license file content

[jhlegarreta]

Sync the license file with the current Slicer license file content: https://github.com/Slicer/Slicer/blob/ce1ab3d3799ca9e11cc1175f4d0c8653d4837e11/License.txt

Specifically:

• Prefer using https over http for web addresses when available.
• Fix a typo following: https://github.com/Slicer/Slicer/commit/68ff0ae7114e4378322740f117643d7513a71110
• Add a blank line at the beginning of the file and remove an unnecessary whitespace in the text.

Make the file have the .txt extension instead of .md and make its name all lowercase to sync with the Slicer license file.