docs: fix grammar - change 'it's' to 'its'

Fixes #589

Changed 'it's' (it is) to 'its' (possessive) in the load_dotenv docstring.

Author: menxia

src/dotenv/main.py

@@ -403,7 +403,7 @@ def load_dotenv(
         Bool: True if at least one environment variable is set else False
 
     If both `dotenv_path` and `stream` are `None`, `find_dotenv()` is used to find the
-    .env file with it's default parameters. If you need to change the default parameters
+    .env file with its default parameters. If you need to change the default parameters
     of `find_dotenv()`, you can explicitly call `find_dotenv()` and pass the result
     to this function as `dotenv_path`.