fix(typing): reword the comment mypy reads as a # type: comment

[Danipulok]

What

mypy cannot check this project at all. One run, on a clean tree:

pyrogram/connection/transport/tcp/web_proxy_carrier.py:40: error: Invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

The file parses fine and every other tool reads it. What mypy chokes on is a comment:

# type:u8 | stream_id:u24 (big-endian) | length:u32 (big-endian) | payload, laid
#  out by tdesktop's `SerializeFrame`.

A comment that begins # type: is a PEP 484 type comment, so mypy parses the rest of the
line as a type expression, fails, and stops. It reports the one syntax error and checks
nothing else in the tree.

The change

The comment says the same thing, starting from a word instead of from type::

# A frame header is `type:u8 | stream_id:u24 (big-endian) | length:u32 (big-endian) |
#  payload`, laid out by tdesktop's `SerializeFrame`. It cannot start at `type:`, which
#  `mypy` reads as a `# type:` comment and refuses to parse.

No code changes. Afterwards:

Found 7492 errors in 1110 files (checked 4119 source files)

That number is what the annotations actually look like today, and it is a separate matter —
this only makes it visible.