Fix incomplete sentence in flow control documentation

[Copilot]

The BDP section had an incomplete sentence ending with "quic-go therefore" and incorrectly stated "receiver won't be able to send" instead of "sender won't be able to send".

Changes:

• Corrected subject from "receiver" to "sender" when describing flow control blocking behavior
• Completed the sentence by adding reference to the auto-tuning algorithm

The sentence now reads:

If the receiver's flow control window is smaller than the BDP, the sender won't be able to send any more data before receiving additional flow control credit, making it impossible to fully utilize the available bandwidth. quic-go therefore implements an auto-tuning algorithm to dynamically adjust the receive window size.

This properly introduces the auto-tuning feature detailed in the subsequent section.

Original prompt

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This section details on the original issue you should resolve

<issue_title>incomplete sentence in flow control section</issue_title>
<issue_description>> should it be "the sender won't be able to send". also, what comes after therefore?? :)

Originally posted by @mwk24 in eabd6c5</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes incomplete sentence in flow control section #99

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