Fix typos with codespell

Using codespell 1.16.0.

Method:
```
$ cat > ../godot-word-whitelist.txt << EOF
ang
curvelinear
dof
doubleclick
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
```

_static/css/custom.css

@@ -143,7 +143,7 @@ code,
 .wy-menu-vertical p.caption {
     color: var(--navbar-heading-color);
 
-    /* Improves the apperance of uppercase text */
+    /* Improves the appearance of uppercase text */
     letter-spacing: 0.75px;
 }