docs: correct optional vs required positional args format

the source arg is always required, identifier is optional.
the opposite was remnant from an earlier version.

README.md

@@ -2,7 +2,7 @@
 
 usage:
 
-    zdoc [-s] [source] <identifier>
+    zdoc [-s] <source> [identifier]
 
 the program searches source code for matching public identifiers,
 printing found types and their doc comments to stdout.

src/main.zig

@@ -72,7 +72,7 @@ pub fn main() !void {
 
 fn usage(prog: []const u8) !void {
     try stderr.print(
-        \\usage: {s} [-s] [source] <identifier>
+        \\usage: {s} [-s] <source> [identifier]
         \\
         \\the program searches source code for matching public identifiers,
         \\printing found types and their doc comments to stdout.