Minor fixes to pynwb-allenSDK page

[matchings]

found small typo in the nwb/sdk databook page (https://allenswdb.github.io/practicalities/allensdk-pyNWB.html):

"The newer physiology datasets are not access using the Allen SDK" should read "The newer physiology datasets are not accessed using the Allen SDK"

also, i think "Instantiating the cache" could use a bit more explanation, and maybe "cache" should be bolded like "manifest" is? the concept of a cache may not be familiar to everyone.

last comment - i think the paragraphs starting with "the newer physiology datasets..." and "while the allenSDK..." could be swapped, to better group the SDK related info and the pynwb related info.