gh-36145: build/pkgs/mathjax/distros/gentoo.txt: Fix

    
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Fixes the build error in CI for 10.2.beta0
https://github.com/sagemath/sage/actions/runs/5992963191/job/16252928425
#step:8:51
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URL: #36145
Reported by: Matthias Köppe
Reviewer(s): Michael Orlitzky

build/pkgs/mathjax/distros/gentoo.txt

@@ -1 +1 @@
->=dev-libs/mathjax-3
+">=dev-libs/mathjax-3"