docs: minor typo in guides/theming.md (#22899)
docs: minor typo in guides/theming.md (#22899)


mmalerba/material2,mmalerba/material2,angular/components,mmalerba/material2,angular/components,josephperrott/material2,DevVersion/material2,crisbeto/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,josephperrott/material2,crisbeto/material2,DevVersion/material2,DevVersion/material2,mmalerba/material2,josephperrott/material2,mmalerba/material2,crisbeto/material2,angular/components,josephperrott/material2,angular/components,DevVersion/material2