update README of local_testnet (#4114)

## Issue Addressed

NA

## Proposed Changes

update the descriptions of README in `scripts/local_testnet`.

## Additional Info

NA

scripts/local_testnet/README.md

@@ -20,7 +20,7 @@ Modify `vars.env` as desired.
 Start a local eth1 ganache server plus boot node along with `BN_COUNT`
 number of beacon nodes and `VC_COUNT` validator clients.
 
-The `start_local_testnet.sh` script takes three options `-v VC_COUNT`, `-d DEBUG_LEVEL` and `-h` for help.
+The `start_local_testnet.sh` script takes four options `-v VC_COUNT`, `-d DEBUG_LEVEL`, `-p` to enable builder proposals and `-h` for help.
 The options may be in any order or absent in which case they take the default value specified.
 - VC_COUNT: the number of validator clients to create, default: `BN_COUNT`
 - DEBUG_LEVEL: one of { error, warn, info, debug, trace }, default: `info`

scripts/local_testnet/start_local_testnet.sh

@@ -28,7 +28,7 @@ while getopts "v:d:ph" flag; do
         echo "Options:"
         echo "   -v: VC_COUNT    default: $VC_COUNT"
         echo "   -d: DEBUG_LEVEL default: info"
-        echo "   -p:             enable private tx proposals"
+        echo "   -p:             enable builder proposals"
         echo "   -h:             this help"
         exit
         ;;