Mark mfrac@bevelled as deprecated

This is not part of MathML Core, has been removed from Firefox and is
not implemented in other browsers. The update was supposed to be done in
https://bugzilla.mozilla.org/show_bug.cgi?id=1587572#c8

Fixes mdn#15359

files/en-us/web/mathml/element/mfrac/index.md

@@ -22,7 +22,7 @@ The MathML `<mfrac>` element is used to display fractions.
 
 This element's attributes include the [global MathML attributes](/en-US/docs/Web/MathML/Global_attributes).
 
-- `bevelled`
+- `bevelled` {{deprecated_inline}}
   - : Specifies the way the fraction is displayed. If `true`, the fraction line is bevelled, which means that numerator and denominator are displayed side by side and separated by a slash (/). Otherwise, if set to `false` (which is the default value), numerator and denominator are on top of each other.
 - `denomalign` {{deprecated_inline}}
   - : The alignment of the denominator under the fraction. Possible values are: `left`, `center` (default), and `right`.