Fix InterProcessReaderWriterLock context manager example in readme

[iver56]

Thanks for making fasteners! 😄

I tried to run code like this (based on what I found in README):

import fasteners

rw_lock = fasteners.InterProcessReaderWriterLock("my_tiny.lock")

with rw_lock.write_locked():
    print("writing")

with rw_lock.read_locked():
    print("reading")

I got an error like

Traceback (most recent call last):
  File "C:/path/to/example.py", line 5, in <module>
    with rw_lock.write_locked():
AttributeError: '_WindowsInterProcessReaderWriterLock' object has no attribute 'write_locked'

Process finished with exit code 1

I looked at the fasteners source code and figured that I should use write_lock instead of write_locked. I hereby propose that the readme should be updated accordingly.

[psarka]

Thanks!