README: Limiting concurrency - fetch symbol instead of string

[BenSto]

Hi,

first of all thanks for the great work, indeed a good job!

I found a minor mistake in the README regarding the concurrency limitation when using a keyword argument. Currently it says to use [] with a string, but the key in the arguments hash is a symbol, so the expression always turns into nil .
Like proposed in this PR it should be a symbol instead of a string and I think it is also better to use .fetch instead of [], so it raises immediately a KeyError instead of turning silently into nil, if the key does not exist.

Feel free to just adapt the README and close this PR.

[bensheldon]

@BenSto thanks for flagging this! I just made a few tweaks and will merge. Thank you! 🎉

[BenSto]

Thank you!

@bensheldon I have to say thank you, first for the fast response and second for the great work.