Correct the licence reference in README.md (#42)

Using the short identifier specified by SPDX
at https://spdx.org/licenses/BSD-2-Clause

Fixes #41

README.md

@@ -65,4 +65,4 @@ Before proposing a change, please discuss your change by raising an issue.
 
 ## Licence
 
-MIT
+BSD-2-Clause