[commands] Fix Paginator still allowing lines which are too long.

discord/ext/commands/formatter.py

@@ -89,7 +89,7 @@ class Paginator:
         RuntimeError
             The line was too big for the current :attr:`max_size`.
         """
-        if len(line) >= self.max_size:
+        if len(line) > self.max_size - len(self.prefix) - 2:
             raise RuntimeError('Line exceeds maximum page size %s' % (self.max_size))
 
         if self._count + len(line) + 1 > self.max_size: