docs(http1): fix misnamed read_buf_exact_size reference in max_buf_size doc

[ump45nose]

What

In src/client/conn/http1.rs, the max_buf_size builder docs say:

Note that setting this option unsets the read_exact_buf_size option.

There is no read_exact_buf_size method. The builder method is read_buf_exact_size (same file, declared just above max_buf_size). This is a word-order slip in a doc cross-reference.

The reciprocal note on read_buf_exact_size already points back to max_buf_size correctly, so only this one line was off.

Why it matters

The wrong name misleads readers who go looking for a read_exact_buf_size builder method that does not exist.

Test plan

Doc-comment-only change; no code paths touched. Verified the correct method name and the h1_read_buf_exact_size / h1_max_buf_size mutual-unset behavior in the same file at the current master.