Replaced n_ prefix with number_of_ in docs/topics/db/queries.txt.

[rfleschenberg]

Shortening number_of to n_ can be a little confusing to people who are not used it. There is no harm in spelling this out.

[kezabelle]

For further context and rationale, this came up on IRC from someone:

Good morning!
What's the meaning of the n_ prefix in the use of F expressions?
[url to docs]
(Is there any?)
[... guesswork/probable explanation given ...]
I see! Thank you, it seemed weird to me.