[DOC] Update Installation index.rst

[SvenJuergens]

Hi seems a small typo in extension:setup, should be extension Name.
Testet in TYPO3 v11, if i use:

ddev exec vendor/bin/typo3 extension:setup --extension=like-it

i got:
[ERROR] Given extensions "like-it" not found in the system.

this is working:
ddev exec vendor/bin/typo3 extension:setup --extension=like_it

[froemken]

Hi,

thank you for your help. I have approved your PR.
"froemken" is my private account :-)
I will merge it next week @sfroemkenjw

Stefan