oss: add API retrival link for weather doc loader (#1311)

Get API from this link easily.

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---------

Co-authored-by: Mason Daugherty <github@mdrxy.com>

Author: arry043

src/oss/python/integrations/document_loaders/weather.mdx

@@ -15,6 +15,7 @@ pip install -qU  pyowm
 ```
 
 ```python
+# Get an [API key](https://home.openweathermap.org/api_keys) first.
 # Set API key either by passing it in to constructor directly
 # or by setting the environment variable "OPENWEATHERMAP_API_KEY".