[du] updating to say install 1dot7 (dagster-io#21967)

Missed this while updating DU. There's a spot where we install the CLI
first and then install Dagster 1.7 via the project, but we gotta probs
update the CLI command too.

docs/dagster-university/pages/dagster-essentials/lesson-2/requirements-and-installation.md

@@ -31,5 +31,5 @@ pip --version
 To install Dagster into your current Python environment:
 
 ```shell
-pip install 'dagster~=1.6'
+pip install 'dagster~=1.7'
 ```