modified max duration error for better understanding

[kapilkd13]

When embedding video larger than the maxduration set, the error thrown suggests to set max_duration param but it is rather maxduration without underscore. Also added a line to suggest in which function to make changes. It is minor, but it had me scratching my head for a minute.

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• I have added a test to the test suite, if necessary
• I have properly documented new or changed features in the documention, or the docstrings
• I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

[kapilkd13]

@Zulko Can you look at this. Its very trivial PR, doesn't affect the core codebase in any way.

[Zulko]

Sure. Sorry for being slow.