Add instruction to reload bash profile

[TheCarpetMerchant]

Page URL

https://docs.flutter.dev/install/manual

Page source

https://github.com/flutter/website/blob/main/src/content/install/manual.md

Describe the problem

bash instruction do not include source ~/.bash_profile

Expected fix

No response

Additional context

No response

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.