Fix typos in the introductory tutorials

Corrections for the typos and minor issues I noticed when reading through the introductory tutorials. I tried to avoid getting into nitpick territory regarding commas and the like, so these should all be clear and evident fixes.

I also added a note regarding the total disk space required for ISE WebPack installation, as that was actually something that I had a problem with; namely, I had to resize my Linux virtual machine's disks to be able to install ISE after it informed me it wanted 16GB space. Good to have a warning about that up front.

Note also issue #1, which precluded typo corrections to the "How does an FPGA work?" page (it does have typos).