Fix DOMPoint»w|x|y|z spec URLs (mdn#11167)

There aren’t actually any #dom-dompoint-w|x|y|z fragment IDs in the
https://drafts.fxtf.org/geometry/ spec. Instead the spec has shared text
that the DOMPointReadOnly and DOMPoint IDLs both link to, at, e.g.,
https://drafts.fxtf.org/geometry/#dom-dompointreadonly-x, which says:

> The x attribute, on getting, must return the x coordinate value. For
> the DOMPoint interface, setting the x attribute must set the x
> coordinate to the new value.

That is, the first sentence of that shared text applies to both the
DOMPointReadOnly and DOMPoint interfaces, while the second sentence
applies only to the DOMPoint interface.

api/DOMPoint.json

@@ -150,7 +150,7 @@
       "w": {
         "__compat": {
           "mdn_url": "https://developer.mozilla.org/docs/Web/API/DOMPoint/w",
-          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompoint-w",
+          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompointreadonly-w",
           "support": {
             "chrome": {
               "version_added": "61"
@@ -247,7 +247,7 @@
       "x": {
         "__compat": {
           "mdn_url": "https://developer.mozilla.org/docs/Web/API/DOMPoint/x",
-          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompoint-x",
+          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompointreadonly-x",
           "support": {
             "chrome": {
               "version_added": "61"
@@ -296,7 +296,7 @@
       "y": {
         "__compat": {
           "mdn_url": "https://developer.mozilla.org/docs/Web/API/DOMPoint/y",
-          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompoint-y",
+          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompointreadonly-y",
           "support": {
             "chrome": {
               "version_added": "61"
@@ -345,7 +345,7 @@
       "z": {
         "__compat": {
           "mdn_url": "https://developer.mozilla.org/docs/Web/API/DOMPoint/z",
-          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompoint-z",
+          "spec_url": "https://drafts.fxtf.org/geometry/#dom-dompointreadonly-z",
           "support": {
             "chrome": {
               "version_added": "61"