Merge #13476: Fix incorrect shell quoting in FreeBSD build instructions.

c9924a2 Fix incorrect shell quoting in FreeBSD build instructions. (murrayn)

Pull request description:

  The current instructions suggest:

      BDB_PREFIX='$PWD/db4'

  which results in BDB_PREFIX being set, literally, to '$PWD/db4'.

Tree-SHA512: 4a920d7a506ff94fb12793f0b8161fdfe0798382cfa88290a0eadd6926843b577328f7159a55039e2655def7ddf2e7a1eaa3dd30490292d4a920c98759becf56

Author: MarcoFalke

doc/build-freebsd.md

@@ -17,7 +17,7 @@ pkg install autoconf automake boost-libs git gmake libevent libtool openssl pkgc
 For the wallet (optional):
 ```
 ./contrib/install_db4.sh `pwd`
-export BDB_PREFIX='$PWD/db4'
+export BDB_PREFIX="$PWD/db4"
 ```
 
 See [dependencies.md](dependencies.md) for a complete overview.