[DialogContent] Add missing dividers class types

[NickCis]

• I have followed (at least) the PR section of the contributing guide.

According to the docs. Dialog Content could receive a dividers attribute in the classes property. Current type definitions only allows for a root attribute, which causes the following error if a dividers attribute in the classes property is passed:

TS2769: No overload matches this call.
  Overload 1 of 2, '(props: DialogContentProps, context?: any): ReactElement<any, string | ((props: any) => ReactElement<any, string | ... | (new (props: any) => Component<any, any, any>)> | null) | (new (props: any) => Component<...>)> | Component<...> | null', gave the following error.
    Type '{ dividers: string; }' is not assignable to type 'Partial<Record<"root", string>>'.
      Object literal may only specify known properties, and 'dividers' does not exist in type 'Partial<Record<"root", string>>'.
  Overload 2 of 2, '(props: PropsWithChildren<DialogContentProps>, context?: any): ReactElement<any, string | ((props: any) => ReactElement<any, string | ... | (new (props: any) => Component<...>)> | null) | (new (props: any) => Component<...>)> | Component<...> | null', gave the following error.
    Type '{ dividers: string; }' is not assignable to type 'Partial<Record<"root", string>>'.
      Object literal may only specify known properties, and 'dividers' does not exist in type 'Partial<Record<"root", string>>'.

This PR adds the dividers attribute to typescript type definition of the classes property.

[mui-pr-bot]

No bundle size changes comparing f7a2c42...0f72a2d

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[oliviertassinari]

@NickCis It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!