Fix Cask name of MPlus [skip ci]

[why]
I am not sure why this is wrong, maybe someone renamed the Cask?

Ah, here it is:
Homebrew/homebrew-cask@87e1c59

[how]
Follow the renaming done by Homebrew.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

bin/scripts/lib/fonts.json

@@ -698,7 +698,7 @@
       "imagePreviewFont": "M+1Code Nerd Font",
       "imagePreviewFontSource": "MPlus/M_Plus_1_code/Mplus1Code-Regular.ttf",
       "linkPreviewFont": "mplus",
-      "caskName": "mplus",
+      "caskName": "m+",
       "repoRelease": true,
       "description": "Multiple styles and weights, many glyph sets (e.g. Kana glyphs)"
     },