docs(material/button-toggle): add missing type on public API (#22224)
docs(material/button-toggle): add missing type on public API (#22224)

Adds an explicit type to `MatButtonToggleGroup` so that it can show up in the docs.
This change is merge safe, because the type is the same as the inferred type that was in
place already.

Fixes #22026.
crisbeto/material2,josephperrott/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,angular/components,angular/components,mmalerba/material2,crisbeto/material2,DevVersion/material2,mmalerba/material2,DevVersion/material2,andrewseguin/material2,andrewseguin/material2,mmalerba/material2,DevVersion/material2,josephperrott/material2,DevVersion/material2,angular/components,josephperrott/material2,angular/components,andrewseguin/material2,mmalerba/material2,andrewseguin/material2,crisbeto/material2,andrewseguin/material2,mmalerba/material2,josephperrott/material2