Fix dots spacing

\cdots, \ddots, and \vdots are all ords, not puncts.

Test plan:
 - Make sure no huxley tests changed

Auditors: alpert

buildTree.js

@@ -30,6 +30,7 @@ var groupToType = {
     text: "mord",
     open: "mopen",
     close: "mclose",
+    inner: "minner",
     frac: "minner",
     spacing: "mord",
     punct: "mpunct",
@@ -243,6 +244,11 @@ var groupTypes = {
             group.value, group.mode, options.getColor(), ["mclose"]);
     },
 
+    inner: function(group, options, prev) {
+        return buildCommon.mathrm(
+            group.value, group.mode, options.getColor(), ["minner"]);
+    },
+
     frac: function(group, options, prev) {
         var fstyle = options.style;
         if (group.value.size === "dfrac") {

symbols.js

@@ -771,17 +771,17 @@ var symbols = {
         },
         "\\cdots": {
             font: "main",
-            group: "punct",
+            group: "inner",
             replace: "\u22ef"
         },
         "\\ddots": {
             font: "main",
-            group: "punct",
+            group: "inner",
             replace: "\u22f1"
         },
         "\\vdots": {
             font: "main",
-            group: "punct",
+            group: "textord",
             replace: "\u22ee"
         }
     },