Reducing the size of the text pool used for comments

Summary:
Every time dbgen starts is take a while to generate a 300MiB text used
for random string generation. I reduced it to 3MiB to speed up the start
time

Test Plan: psyduck

Reviewers: adam, alex, anders, ankur, arthur, chase, dolphinigle, jvictor, manu, marko, mom, nika, nikita, pieguy, rumen

CC: engineering

Maniphest Tasks: