Make Kotlin collection example clearer (#12)

* Make Kotlin collection example clearer

* Use val over var in collection example

Author: russellbanks

code/collections.kt

@@ -1,17 +1,15 @@
-var datas = listOf(
+val datas = listOf(
     SensorData(1, "A", 2.89),
     SensorData(2, "B", 12.01),
     SensorData(3, "B", 11.89),
     SensorData(4, "A", 3.11),
     SensorData(5, "A", -456.0)
 )
 
-var avgs = datas
-            .filter { it.value > -50.0 }
-            .groupBy { it.location }
-            .map { g -> 
-                Location(g.key, 
-                         g.value.map { it.value }.average()) }
-
+val avgs = datas
+    .filter { it.value > -50.0 }
+    .groupBy(SensorData::location)
+    .map { Location(it.key, it.value.map(SensorData::value).average()) }
+    
 // (location=A, value=3.0)
 // (location=B, value=11.95)

index.html

@@ -398,20 +398,18 @@
 var multiplied = numbers.Select(e => 3 * e);
 // [ 60, 57, 21, 36 ]</code></pre></div></div></div><div class="case"><div class="name">Sort</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val ordered = listOf(1, 5, 3, 12, 2).sorted()
 // [ 1, 2, 3, 5, 12 ]</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var ordered = new[] { 1, 5, 3, 12, 2 }.OrderBy(i => i);
-// [ 1, 2, 3, 5, 12 ]</code></pre></div></div></div><div class="case"><div class="name">Filter / GroupBy / Average</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>var datas = listOf(
+// [ 1, 2, 3, 5, 12 ]</code></pre></div></div></div><div class="case"><div class="name">Filter / GroupBy / Average</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val datas = listOf(
     SensorData(1, "A", 2.89),
     SensorData(2, "B", 12.01),
     SensorData(3, "B", 11.89),
     SensorData(4, "A", 3.11),
     SensorData(5, "A", -456.0)
 )
 
-var avgs = datas
-            .filter { it.value > -50.0 }
-            .groupBy { it.location }
-            .map { g -> 
-                Location(g.key, 
-                         g.value.map { it.value }.average()) }
+val avgs = datas
+    .filter { it.value > -50.0 }
+    .groupBy(SensorData::location)
+    .map { Location(it.key, it.value.map(SensorData::value).average()) }
 
 // (location=A, value=3.0)
 // (location=B, value=11.95)</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var datas = new List&lt;SensorData>