update to activate

docs/Installation-Windows.md

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-To use this environment, you will need to activate it.  _(In the future, if you need to use this environment again, you can run the same command)_.  In the same Anaconda Prompt, type in the following command:
+To use this environment, you must activate it. _(To use this environment In the future, you can run the same command)_. In the same Anaconda Prompt, type in the following command:
 
 ```
 conda activate ml-agents