Fix small error with example command for e2e tests

[cpilsworth]

Description

aio app test -e tries to run tests on an extension only, so the example gives the error:

Error: Flag --extension expects a value

aio app test --e2e will run the end-to-end tests as expected

Related Issue

Closes #182

Motivation and Context

Fixes a minor issue with the generated README file that might slow down or frustrate new users of the platform.

How Has This Been Tested?

Tested updated command locally with the updated command, with reference to the command help.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #183 (bd53167) into master (100abd2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #183   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          645       645           
  Branches        71        71           
=========================================
  Hits           645       645           

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[shazron]

thank you @cpilsworth !