Fix a minor grammar error in some docstrings.

it's means "it is". its is the possessive.

libcst/_nodes/base.py

@@ -120,7 +120,7 @@ def __post_init__(self) -> None:
  def __init_subclass__(cls, **kwargs: Any) -> None:
  """
  HACK: Add our implementation of `__repr__`, `__hash__`, and `__eq__` to the
- class's __dict__ to prevent dataclass from generating it's own `__repr__`,
+ class's __dict__ to prevent dataclass from generating its own `__repr__`,
  `__hash__`, and `__eq__`.
 
  The alternative is to require each implementation of a node to remember to add
@@ -152,7 +152,7 @@ def validate_types_shallow(self) -> None:
  Compares the type annotations on a node's fields with those field's actual
  values at runtime. Raises a TypeError is a mismatch is found.
 
- Only validates the current node, not any of it's children. For a recursive
+ Only validates the current node, not any of its children. For a recursive
  version, see :func:`validate_types_deep`.
 
  If you're using a static type checker (highly recommended), this is useless.