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
diff --git a/README.md b/README.md
index 236960d..b2f741a 100644
--- a/README.md
+++ b/README.md
@@ -65,4 +65,4 @@
 
 ## Licence
 
-MIT
+BSD-2-Clause