Two small spelling mistakes in the tutorials

[Russell Haley]

Two small spelling mistakes in the tutorials:
https://github.com/Yonaba/Moses/blob/master/doc/tutorial.md

pluck (t, property)
Fetches all values indxed with specific key in a table of objects.
indxed ->indexed

constant (value)
Creates a constant function. This function will constinuously yield the same output.
constinuously -> continuously

Cheers,
Russ

[Roland]

Much thanks, @RussellHaley !