Update "setting up your environment" in CONTRIBUTING.md #913 (#918)

Author: tawandamoyo

CONTRIBUTING.md

@@ -242,7 +242,7 @@ conda env create
 Before building and testing the project, you have to activate the environment:
 
 ```bash
-source activate ENVIRONMENT_NAME
+conda activate pygmt
 ```
 
 You'll need to do this every time you start a new terminal.