docs(material/stepper): fix typo into stepper documentation (#24618)
docs(material/stepper): fix typo into stepper documentation (#24618)

Fix a small typo error at Material Stepper documentation.

Fixes #24613
crisbeto/material2,angular/components,angular/components,angular/components,crisbeto/material2,angular/components,crisbeto/material2,crisbeto/material2