Update skewdata_reduce.ipynb

_doc/notebooks/expose/skewdata_reduce.ipynb

@@ -156,7 +156,7 @@
         "\n",
         "$$C= K \\sum_i \\frac{f(i)}{i^s}$$\n",
         "\n",
-        "The cost $f(i)$ is usually polynomial: $f(i) \\propto i^t$. Then $C \\propto \\sum_i i^{t-s}$. This sum can be computed only if $t<s-1$. \n",
+        "The cost $f(i)$ is usually polynomial: $f(i) \\propto i^t$. Then $C \\propto \\sum_i i^{t-s}$. This sum can be computed only if $t<s-1$. \n",
         "\n",
         "**What does it means?**\n",
         "\n",