Change how tree[subscript] is introduced

This modifies the Tree docstring to replace the phrase "Tree as a
list" with a phrase that:

- Encompasses the use where subscripts are not integers or slices.
- Is more precise in its terminology.

The former advantage--not leaving out the case where one can pass
a string--is the rationale.

Author: EliahKagan

git/objects/tree.py

@@ -164,7 +164,7 @@ class Tree(IndexObject, git_diff.Diffable, util.Traversable, util.Serializable):
     R"""Tree objects represent an ordered list of :class:`~git.objects.blob.Blob`\s and
     other :class:`Tree`\s.
 
-    Tree as a list:
+    Subscripting is supported, as with a mapping or sequence:
 
     * Access a specific blob using the ``tree["filename"]`` notation.
     * You may likewise access by index, like ``blob = tree[0]``.