Fix issue probably-meant-fstring found at https://codereview.doctor (a…

…io-libs#6718)

aiohttp/client_reqrep.py

@@ -215,7 +215,7 @@ def __init__(
         if match:
             raise ValueError(
                 f"Method cannot contain non-token characters {method!r} "
-                "(found at least {match.group()!r})"
+                f"(found at least {match.group()!r})"
             )
         assert isinstance(url, URL), url
         assert isinstance(proxy, (URL, type(None))), proxy