fix: fixed typo with experimental_approximateDocumentStoreMiB parameter

[Arfey]

this parameter didn't use even user specify it. i just initialize it in constructor. Also could u explain this comment:

// Create ~about~ a 30MiB InMemoryLRUCache.  This is less than precise
      // since the technique to calculate the size of a DocumentNode is
      // only using JSON.stringify on the DocumentNode (and thus doesn't account
      // for unicode characters, etc.), but it should do a reasonable job at
      // providing a caching document store for most operations.

Math.pow(2, 20) * (this.experimental_approximateDocumentStoreMiB || 30)

why is it around 30MiB? we have 30 million keys in lru-cache. I'm not sure that it's true (because each key must be equel 1b).

[glasser]

We fixed this via a different PR (#5629). Sorry for the delay!