Use plain inline code for LineTooLong in CHANGES entry

Sphinx nitpicky mode can't resolve :exc:`~aiohttp.http_exceptions.LineTooLong`
since the exception isn't autodoc'd. Match the convention from other CHANGES
entries (e.g. 10611.bugfix.rst) and use plain inline code instead.

Author: yuj

CHANGES/7177.bugfix.rst

@@ -1,6 +1,6 @@
-Improved the :exc:`~aiohttp.http_exceptions.LineTooLong` error message to
-describe which part of the HTTP message exceeded the limit (e.g. the request
-URL, status reason phrase, header field name/value, request/status line, or
-trailer). Previously, every overflow reported a generic ``"when reading:"``
-prefix, which was confusing when only the URL was too long but the message was
-historically labeled as a status-line overflow -- by :user:`Ricardo-M-L`.
+Improved the ``LineTooLong`` error message to describe which part of the HTTP
+message exceeded the limit (e.g. the request URL, status reason phrase, header
+field name/value, request/status line, or trailer). Previously, every overflow
+reported a generic ``"when reading:"`` prefix, which was confusing when only
+the URL was too long but the message was historically labeled as a status-line
+overflow -- by :user:`Ricardo-M-L`.