docs: typo: tiny grammar change: "pointed by" -> "pointed to by" (#118411)

* docs: tiny grammar change: "pointed by" -> "pointed to by"

This commit uses "file pointed to by" to replace "file pointed by" in
 - doc for shutil.copytree
 - docstring for shutil.copytree
 - docstring _abc.PathBase.open
 - docstring for pathlib.Path.open
 - doc for os.copy_file_range
 - doc for os.splice

The docs use "file pointed to by" more frequently than
"file pointed by". So, this commit replaces the uses of
"file pointed by" in order to make the uses consistent
through the docs.

```bash
$ grep -ri 'pointed to by' cpython/
```
yields more results than
```bash
$ grep -ri 'pointed by' cpython/
```

Separately:

There are two occurrences of "tree pointed by":
 - cpython/Doc/library/xml.etree.elementtree.rst for
     `xml.etree.ElementInclude.include`
 - cpython/Lib/xml/etree/ElementInclude.py for `include`

For those uses of "tree pointed by", I expect "tree pointed to by"
instead. However, I found enough uses online of (a) "tree pointed by"
rather than (b) "tree pointed to by" to convince me that (a) is in
common use.

So, this commit does not replace those occurrences of "tree pointed by"
to "tree pointed to by". But I will replace them if a reviewer
believes it is correct to replace them.

* docs: typo: "exists and executable" -> "exists and is executable"

---------

Co-authored-by: Andrew-Zipperer <atzipperer@gmail.com>