[Python] Fix incorrectly capitalized mustache identifier in apis template

Created by: harmony7

This tiny PR fixes the spelling of an identifier that was incorrectly cased in a mustache template. The fix only affects a comment in the output, so it will make no runtime difference when consuming a generated client.

In the Python generator, the package name is available as {{packageName}}, but in src/main/resources/python/__init__apis.mustache, it was being referenced as {{packagename}}. This produced incorrect output such as:

# In order to avoid this, import only the API that you directly need like:
#
#   from .api.my_api import MyApi

(note the missing package name)

The corrected output:

# In order to avoid this, import only the API that you directly need like:
#
#   from my_package.api.my_api import MyApi

cc @cliffano @zlx @autopp

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