Fix wording in 'Understanding the Controller Component' doc

[Nitrodist]

Fixes the wording so that it goes from:

'Do not use controllers for to run stateless or stateful code shared across controllers'

to:

'Do not use controllers for stateless or stateful code sharing across controllers'

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[Nitrodist]

Signed the CLA as Mark Campbell and updated the commit message.

[petebacondarwin]

Thanks Mark