docs: missing 'b' in python command (#3728)

In Python 3 the command outlined in this doc `import codecs; codecs.decode("YWJjZA==", 'base64').decode('ascii')` throws an error:
TypeError: decoding with 'base64' codec failed (TypeError: expected bytes-like object, not str), needs to add 'b' before the encoded string
`import codecs; codecs.decode(b"YWJjZA==", 'base64').decode('ascii')` to make it work

docs/app-dev/getting-started.md

@@ -137,7 +137,7 @@ The result should look like:
 Note the `value` in the result (`YWJjZA==`); this is the base64-encoding
 of the ASCII of `abcd`. You can verify this in a python 2 shell by
 running `"YWJjZA==".decode('base64')` or in python 3 shell by running
-`import codecs; codecs.decode("YWJjZA==", 'base64').decode('ascii')`.
+`import codecs; codecs.decode(b"YWJjZA==", 'base64').decode('ascii')`.
 Stay tuned for a future release that [makes this output more
 human-readable](https://github.com/tendermint/tendermint/issues/1794).