Fix some type error

Author: Yibo-Chen13

proton_driver/columns/nestedcolumn.py

@@ -4,7 +4,7 @@
 
 def create_nested_column(spec, column_by_spec_getter):
     return create_array_column(
-        'Array(Tuple({}))'.format(','.join(get_nested_columns(spec))),
+        'array(tuple({}))'.format(','.join(get_nested_columns(spec))),
         column_by_spec_getter=column_by_spec_getter
     )
 
@@ -58,7 +58,7 @@ def get_columns_with_types(spec):
 
 def get_inner_spec(spec):
     brackets = 0
-    offset = len('Nested')
+    offset = len('nested')
     i = offset
     for i, ch in enumerate(spec[offset:], offset):
         if ch == '(':

proton_driver/columns/numpy/lowcardinalitycolumn.py

@@ -90,6 +90,6 @@ def _read_data(self, n_items, buf, nulls_map=None):
 
 
 def create_numpy_low_cardinality_column(spec, column_by_spec_getter):
-    inner = spec[15:-1]
+    inner = spec[16:-1]
     nested = column_by_spec_getter(inner)
     return NumpyLowCardinalityColumn(nested)

proton_driver/columns/numpy/stringcolumn.py

@@ -73,6 +73,6 @@ def create_string_column(spec, column_options):
         cls = NumpyByteStringColumn if strings_as_bytes else NumpyStringColumn
         return cls(encoding=encoding, **column_options)
     else:
-        length = int(spec[12:-1])
+        length = int(spec[13:-1])
         cls = NumpyByteFixedString if strings_as_bytes else NumpyFixedString
         return cls(length, encoding=encoding, **column_options)