Merge bitcoin#17751: doc: use recommended shebang approach in documentation code block

6094222 use preferred shebang approach for documentation (hackerrdave)

Pull request description:

  Documentation update to use recommended shebang approach mentioned in the [developer notes](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#shebang)

ACKs for top commit:
  hebasto:
    ACK 6094222, I have reviewed the code, and it looks OK, I agree it can be merged.

Tree-SHA512: fc58632f0a6fa82c7abdddfac4897f082110d647426d2b468cba6fabf6b34a015fcad47e5b26be98e629b8b0417b8781e8d89da67189e20da228b97b17f1a532

Author: MarcoFalke

doc/reduce-memory.md

@@ -41,7 +41,7 @@ threads take up 8MiB for the thread stack on a 64-bit system, and 4MiB in a
 By default, since glibc `2.10`, the C library will create up to two heap arenas per core. This is known to cause excessive memory usage in some scenarios. To avoid this make a script that sets `MALLOC_ARENA_MAX` before starting bitcoind:
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 export MALLOC_ARENA_MAX=1
 bitcoind
 ```