Update default.spec

[RobertFlatt]

In buildozer.spec

# (str) python-for-android git clone directory (if empty, it will be automatically cloned from github)
# p4a.source_dir = ../python-for-android

p4a is not automatically cloned. Log message is

# Install platform
# Failed to read python-for-android setup.py at ../python-for-android/setup.py

Change doc string to

# (str) python-for-android git clone directory

[akshayaurora]

👍