Convert equation to math markup

since pkgdown will now take care of the mathml rendering

vignettes/renormalise.Rmd

@@ -27,16 +27,9 @@ as well as the conditions during the recording (including ambient light,
 temperature, *etc*.). To allow for comparison between studies, it is thus 
 normalised  by a white and a dark reference with the following formula:
 
-<math>
-    <mrow>
-      <mtext>Processed</mtext>
-      <mo>=</mo>
-      <mfrac>
-        <mrow><mtext>Raw</mtext><mo>-</mo><mtext>Dark</mtext></mrow>
-        <mrow><mtext>White</mtext><mo>-</mo><mtext>Dark</mtext></mrow>
-      </mfrac>
-    </mrow>
-</math>
+$$
+\dfrac{\text{Raw} - \text{Dark}}{\text{White} - \text{Dark}}
+$$
 
 For this example here, we need to normalise the raw data by a white reference
 contained in another file. This can't be done with with `lr_get_spec()` because