Update install.md Windows PowerShell installation example is actually two commands.

[dezdez]

Removed erroneous backtick character. This is two separate PowerShell commands.

Closes #

Describe your proposed changes here.

• [x] Signed the InfluxData CLA (if necessary)
• [x] Rebased/mergeable

[telegraf-tiger[bot]]

Thanks so much for the pull request! :handshake: :black_nib: Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[dezdez]

!signed-cla