chore: Update SPIS03.ipynb with rectangular window explanation

03-Fourier-Transform/SPIS03.ipynb

@@ -691,6 +691,7 @@
       "source": [
         "### The rectangular window\n",
         "Let us first look at the rectangular window\n",
+        "\n",
         "$$\n",
         "    w_n = \\begin{cases}\n",
         "        1 & n=0,1,\\ldots, N-1\\\\\n",
@@ -710,6 +711,7 @@
       },
       "source": [
         "We can think of the rectangular window $w_n$ as a way of **extracting the samples we see** from an infinitely long signal $x_n$, i.e.,\n",
+        "\n",
         "$$\n",
         "    w_nx_n \\quad\\text{for } n=-\\infty,\\ldots,\\infty\\ .\n",
         "$$\n",