Update copyright for documentation

Signed-off-by: Sean Young <sean@mess.org>

docs/conf.py

@@ -18,7 +18,7 @@
 # -- Project information -----------------------------------------------------
 
 project = 'Solang Solidity Compiler'
-copyright = '2019, 2020 Sean Young <sean@mess.org>'
+copyright = '2019 - 2021 Sean Young <sean@mess.org>'
 author = 'Sean Young <sean@mess.org>'
 
 # The full version, including alpha/beta/rc tags