Wrong spelling in the hook_search_execute() documentation

This is not what I meant with "very shortly." I had to take care of family things. :-)

This patch only fixes the typo.

I added the article for consistency with the other sentence, "The functions search_expression_extract() and search_expression_insert() may also be helpful for adding custom search parameters to the search expression." I guess that using the article depends on how much definite is the method being mentioned. Is the method defined by a class that implements an interface, or is it the method defined by a specific class?
To me, "use function abc()" sounds like it is missing the article, and it sounds like saying "use coffee" instead of "use the coffee you find on the counter"; both are valid, but the latter is referring to specific coffee (i.e., it's not the coffee that you find in another place).

This is secondary, though. We could also argue that the other sentence can be simply written "search_expression_extract() and search_expression_insert() may also be helpful for adding custom search parameters to the search expression," as it should be implicit that search_expression_extract() and search_expression_insert() are functions, and not methods. (It is implicit by the name, and for the fact I don't write "the methods" before them.)