docs(chips): fix autocomplete-chips example

[julianobrasil]

Fixes #11291.

Although #11233 fixes the issue that prevented the chips removal, #11291 points to another issue: when the user pressed enter (or selected a chip from the autocomplete panel) to create the new chip, the autocomplete associated input wasn't cleared appropriately: it kept the last typed/selected item.

Current Behavior:

After fixing it

Fix suggested by @CurtisDS.

[crisbeto]

LGTM

[ngbot]

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