Small changes to memory doc (#2062)

* Small changes to memory doc
- Fixed typo "except" => "accept"
- Use `Counter` explicitly in ROM section example.

* Fix counter doc compile error

* remove invisible doc block in memory example

* more small fixes to make mem example pass doc compile

* Get rid of sine wave iterator in memory doc

* get rid of tabs on VecInit example

* get rid of tabs on VecInit example