Change \undertilde to \utilde (#948)

* Change \undeertilde to \utilde

In PR #670, I made an error. The function that should have been `\utilde` I named instead `\undertilde`.

There is an `\undertilde` from the `wsuipa` package, but it is a text-mode non-stretchy function. The proper command name is `\utilde`, a math-mode, stretchy function from packge `undertilde`.

This PR fixes my error.

* Fix screenshotter test

src/functions.js

@@ -445,7 +445,7 @@ defineFunction([
 // Stretchy accents under the body
 defineFunction([
     "\\underleftarrow", "\\underrightarrow", "\\underleftrightarrow",
-    "\\undergroup", "\\underlinesegment", "\\undertilde",
+    "\\undergroup", "\\underlinesegment", "\\utilde",
 ], {
     numArgs: 1,
 }, function(context, args) {

src/stretchy.js

@@ -17,7 +17,7 @@ import type {span} from "./domTree";
 const stretchyCodePoint: {[string]: string} = {
     widehat: "^",
     widetilde: "~",
-    undertilde: "~",
+    utilde: "~",
     overleftarrow: "\u2190",
     underleftarrow: "\u2190",
     xleftarrow: "\u2190",
@@ -168,7 +168,7 @@ const svgSpan = function(group: ParseNode, options: Options): span {
     let svgNode;
     let span;
 
-    if (utils.contains(["widehat", "widetilde", "undertilde"], label)) {
+    if (utils.contains(["widehat", "widetilde", "utilde"], label)) {
         // There are four SVG images available for each function.
         // Choose a taller image when there are more characters.
         const numChars = groupLength(group.value.base);