Abbreviations list: make searchable

The abbreviations tsv file

https://github.com/Subaru-PFS/doc/blob/master/misc/PFS_Abbreviations.tsv

contains double quotes (") which prevent GitHub from rendering the document searchable, which is a useful feature. See

https://github.com/isaacs/github/issues/848

for example.

The simplest solution is to change the double-quote to single quotes (') where possible.