polish doc

Author: sycai

bigframes/core/blocks.py

@@ -279,7 +279,7 @@ def label_to_col_id(self) -> typing.Mapping[Label, typing.Sequence[str]]:
     def resolve_label_exact(self, label: Label) -> Optional[str]:
         """Returns the column id matching the label if there is exactly
         one such column. If there are multiple columns with the same name,
-        raises an error. If there is no such column, returns None."""
+        raises an error. If there is no such a column, returns None."""
         matches = self.label_to_col_id.get(label, [])
         if len(matches) > 1:
             raise ValueError(
@@ -290,7 +290,7 @@ def resolve_label_exact(self, label: Label) -> Optional[str]:
     def resolve_label_exact_or_error(self, label: Label) -> str:
         """Returns the column id matching the label if there is exactly
         one such column. If there are multiple columns with the same name,
-        raises an error. If there is no such column, raises an error too."""
+        raises an error. If there is no such a column, raises an error too."""
         col_id = self.resolve_label_exact(label)
         if col_id is None:
             raise ValueError(f"Label {label} not found. {constants.FEEDBACK_LINK}")