Print the usage if no args ar provided to the build_podspecs script (#1665)

Motivation:

The information printed when invoking `build_podspecs.sh` without
arguments is outdated.

Modifications:

- Print usage information when invoking without arguments.

Result:

- 'build_podspecs.sh' is more helpful when you can't remember how it
  should be invoked

scripts/build_podspecs.sh

@@ -44,7 +44,7 @@ done
 shift "$((OPTIND-1))"
 
 if [[ $# -eq 0 ]]; then
-  echo "Must provide target version"
+  usage
   exit 1
 fi