Fixed spelling mistakes in libcudf documentation

[karthikeyann]

ran spell check and made some corrections.

[karthikeyann]

rerun tests

[davidwendt]

Technically behaviour is an acceptable spelling
https://grammarist.com/spelling/behavior-behaviour/
Regardless, it is good to have them all the same.
Thanks for doing this.

[codecov]

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@@               Coverage Diff               @@
##             branch-21.08    #8664   +/-   ##
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  Coverage                ?   10.63%           
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  Files                   ?      109           
  Lines                   ?    18657           
  Branches                ?        0           
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  Hits                    ?     1985           
  Misses                  ?    16672           
  Partials                ?        0           

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[karthikeyann]

@gpucibot merge