Fix collector doc

[ChengYen-Tang]

Description

修正文件中的一些錯字

Motivation and Context

如果 qlib.init(provider_uri=provider_uri, region="CN") 中的 region 使用大寫，會出現以下錯誤

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-c309f40f6ae3> in <module>
      7     from get_data import GetData
      8     GetData().qlib_data(target_dir=provider_uri, region="CN")
----> 9 qlib.init(provider_uri=provider_uri, region="CN")

~/.local/lib/python3.8/site-packages/pyqlib-0.6.3.99-py3.8-linux-x86_64.egg/qlib/__init__.py in init(default_conf, **kwargs)
     24     logger = get_module_logger("Initialization", level=logging.INFO)
     25 
---> 26     C.set(default_conf, **kwargs)
     27 
     28     # check path if server/local

~/.local/lib/python3.8/site-packages/pyqlib-0.6.3.99-py3.8-linux-x86_64.egg/qlib/config.py in set(self, default_conf, **kwargs)
    277 
    278         self.set_mode(default_conf)
--> 279         self.set_region(kwargs.get("region", self["region"] if "region" in self else REG_CN))
    280 
    281         for k, v in kwargs.items():

~/.local/lib/python3.8/site-packages/pyqlib-0.6.3.99-py3.8-linux-x86_64.egg/qlib/config.py in set_region(self, region)
    230     def set_region(self, region):
    231         # raise KeyError
--> 232         self.update(_default_region_config[region])
    233 
    234     def resolve_path(self):

KeyError: 'CN'

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[ghost]

All CLA requirements met.

[you-n-g]

@ChengYen-Tang
It looks great! Thansk for your PR.