Update fetch-params.sh

making sure we put params to correct dir on Mac OS X.
got it from here >> https://github.com/z-classic/zclassic/blob/master/zcutil/fetch-params.sh

zcutil/fetch-params.sh

@@ -2,7 +2,11 @@
 
 set -eu
 
-PARAMS_DIR="$HOME/.zcash-params"
+if [[ "$OSTYPE" == "darwin"* ]]; then
+    PARAMS_DIR="$HOME/Library/Application Support/ZcashParams"
+else
+    PARAMS_DIR="$HOME/.zcash-params"
+fi
 
 SPROUT_PKEY_NAME='sprout-proving.key'
 SPROUT_VKEY_NAME='sprout-verifying.key'