vim-patch:c1c3b5d: runtime(doc): remove unnecessary "an" (#32865)

"umask" is pronounce like "youmask", so having an "an" before it is a
bit strange.  In other places in the help, "umask" is not preceded by
either "a" or "an", and sometimes preceded by "the".

Also, "Note" is usually followed either by ":" or "that" in builtin.txt,
so add a ":" after "Note".

closes: 16860

c1c3b5d6a0

src/nvim/eval.lua

@@ -7551,7 +7551,7 @@ M.funcs = {
       the new directory.  The default is 0o755 (rwxr-xr-x: r/w for
       the user, readable for others).  Use 0o700 to make it
       unreadable for others.  This is used for the newly created
-      directories.  Note an umask is applied to {prot} (on Unix).
+      directories.  Note: umask is applied to {prot} (on Unix).
       Example: >vim
       	call mkdir($HOME .. "/tmp/foo/bar", "p", 0o700)