Update the license template so GitHub can detect it

[oprypin]

The license remains BSD 3-Clause but with a better known text.

Before:

$ ~/.local/share/gem/ruby/3.0.0/bin/licensee detect LICENSE.md
License:        NOASSERTION
Matched files:  LICENSE.md
LICENSE.md:
  Content hash:  da8d25a918e5eada76faf0acaf66e9d9f497f1cf
  License:       NOASSERTION
  Closest non-matching licenses:
    BSD-3-Clause similarity:        97.91%
    BSD-4-Clause similarity:        86.35%
    BSD-3-Clause-Clear similarity:  84.48%

After:

$ ~/.local/share/gem/ruby/3.0.0/bin/licensee detect LICENSE.md
License:        BSD-3-Clause
Matched files:  LICENSE.md
LICENSE.md:
  Content hash:  a961b19cc6921d510e29a13b0ba1a826fcffe41c
  Attribution:   Copyright 2007, 2008 The Python Markdown Project (v. 1.7 and later) 
Copyright 2004, 2005, 2006 Yuri Takhteyev (v. 0.2-1.6b) 
Copyright 2004 Manfred Stienstra (the original version)
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       BSD-3-Clause

---

• Closes change: set LICENSE to from BSD to BSD-3-Clause #1435