chore: make cirtuit diagram with white bg

So that it looks fine on dark themes also

Author: sanjayankur31

Tutorial/_media/equivalentCircuitWhiteBg.png

@@ -99,7 +99,7 @@
     "7) The bottom plot shows the **neural membrane potential activity, V<sub>m</sub>**. The spikes here are called \"action potentials\". This variable is governed by the expression: **dV<sub>m</sub>/dt = (I<sub>inj</sub> - I<sub>Na</sub> - I<sub>K</sub> - I<sub>L</sub>)/C<sub>m</sub>**\n",
     "\n",
     "\n",
-    "![hh](../../Tutorial/_media/equivalentCircuit.png)\n"
+    "![hh](../../Tutorial/_media/equivalentCircuitWhiteBg.png)\n"
    ]
   },
   {