Corrected minor issues in documentation

[jameslamb]

Thank you for this excellent package! It has made my code more powerful and eliminated several hacky workarounds I'd written before discovering bigmemory.

To show my appreciation, I ran devtools:spell_check() on the documentation and found a few small spelling errors. Please consider this PR to address them.

[coveralls]

Coverage increased (+2.9%) to 59.1% when pulling 3aeb847 on jameslamb:misc/docs into 92928b9 on kaneplusplus:master.

[kaneplusplus]

Thanks very much!