Merge #13844: doc: correct the help output for -prune

Summary:
312ff01ee533fab68348283200eb57e9956fdb34 -prune option -help output aligned with code (Hennadii Stepanov)

Pull request description:

The -help output for -prune is aligned with the code.

In the code (.../src/init.cpp#L1063):
```
        if (nPruneTarget < MIN_DISK_SPACE_FOR_BLOCK_FILES) {
            return InitError(strprintf(_("Prune configured below the minimum of %d MiB.  Please use a higher number."), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
        }
```
So correct value of nPruneTarget is **greater than or equal to** MIN_DISK_SPACE_FOR_BLOCK_FILES.

Tree-SHA512: 8e55aa99c8f5a9d020677b0f1b016215e2dbda5fa4ee7c8504b12a3abef226bc21beca118fa332c0bf206a4aff913a5a717b55bb5b2ecdba38423e9c0161209e

Backport of Core PR13844
https://github.com/bitcoin/bitcoin/pull/13844/

The relevent prune code is in init.cpp:1688 for our code base.

Test Plan:

make check
./bitcoind --help

Verify the help text reflects changes.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4775