Update command name | README.md

UPDATE: command name 'serve' to 'start'.
Severity: Low
Signed-off-by: Puneet Verma <puneet@saanvis.com>

README.md

@@ -65,9 +65,9 @@ Once the `burrow.toml` has been created, we run:
 
 ```
 # To select our validator address by index in the GenesisDoc
-burrow serve --validator-index=0
+burrow start --validator-index=0
 # Or to select based on address directly (substituting the example address below with your validator's):
-burrow serve --validator-address=BE584820DC904A55449D7EB0C97607B40224B96E
+burrow start --validator-address=BE584820DC904A55449D7EB0C97607B40224B96E
 ```
 
 and the logs will start streaming through.