Fix typos with codespell

Using codespell 2.0.0.

Method:
```
$ cat > ../godot-word-whitelist.txt << EOF
ang
curvelinear
dof
doubleclick
fave
findn
GIRD
leapyear
lod
merchantibility
nd
numer
ois
ony
que
seeked
synching
te
uint
unselect
webp
EOF

$ codespell -w -q 3 -I ../godot-word-whitelist.txt --skip="./thirdparty,*.po"
$ git diff // undo unwanted changes
```

doc/classes/MeshDataTool.xml

@@ -15,7 +15,7 @@
 		mdt.create_from_surface(mesh, 0)
 		for i in range(mdt.get_vertex_count()):
 		    var vertex = mdt.get_vertex(i)
-		    # In this example we extend the mesh by one unit, which results in seperated faces as it is flat shaded.
+		    # In this example we extend the mesh by one unit, which results in separated faces as it is flat shaded.
 		    vertex += mdt.get_vertex_normal(i)
 		    # Save your change.
 		    mdt.set_vertex(i, vertex)
@@ -33,7 +33,7 @@
 		for (var i = 0; i < mdt.GetVertexCount(); i++)
 		{
 		    Vector3 vertex = mdt.GetVertex(i);
-		    // In this example we extend the mesh by one unit, which results in seperated faces as it is flat shaded.
+		    // In this example we extend the mesh by one unit, which results in separated faces as it is flat shaded.
 		    vertex += mdt.GetVertexNormal(i);
 		    // Save your change.
 		    mdt.SetVertex(i, vertex);