Correct typo in the Iterator adapters section
Correct tense
auto merge of #17511 : MatejLach/rust/iter_guide_typo, r=alexcrichton

The sentence "The new iterator `filter()` produces returns only the elements that that closure returned `true` for:"  can be structured as:

"The new iterator `filter()` produces only the elements that that closure returned `true` for:"

or as:

"The new iterator `filter()` returns only the elements that that closure returned `true` for:"

however, not both. 

I went with "produces", since it then talks about returning true and having "return" so close together doesn't sound nice. 
r @steveklabnik ?