Replace triple backtick in exec_run documentation which caused a rendering error.

Signed-off-by: Adam Dangoor <adamdangoor@gmail.com>

docker/models/containers.py

@@ -179,7 +179,7 @@ class Container(Model):
             (ExecResult): A tuple of (exit_code, output)
                 exit_code: (int):
                     Exit code for the executed command or ``None`` if
-                    either ``stream```or ``socket`` is ``True``.
+                    either ``stream`` or ``socket`` is ``True``.
                 output: (generator, bytes, or tuple):
                     If ``stream=True``, a generator yielding response chunks.
                     If ``socket=True``, a socket object for the connection.