chore: upgrade mathjax to 2.7.5

Signed-off-by: BoHong Li <raccoon@hackmd.io>

Author: a60814billy

package.json

@@ -93,7 +93,7 @@
     "markdown-it-sub": "~1.0.0",
     "markdown-it-sup": "~1.0.0",
     "markdown-pdf": "~9.0.0",
-    "mathjax": "~2.7.0",
+    "mathjax": "~2.7.5",
     "mattermost-redux": "~5.9.0",
     "mermaid": "~8.2.3",
     "method-override": "~2.3.7",