text: Title: Typo in docs
  Body: **Migrated issue, originally created by Tim Mitchell**
  
  https://alembic.readthedocs.org/en/latest/api/autogenerate.html#fine-grained-autogenerate-generation-with-rewriters
  The code sample has
  
  ```
  from alembic import ops
  
  ```
  instead of
  
  ```
  from alembic.operations import ops
  ```
  also the second argument to AlterColumnOp should be op.column.name not op.column_name
  
  