vo_opengl: fix "win" backend name

Although the source file is named w32.c, the backend name was "win"
until recently. It was accidentally changed to "w32"; fix it.

Fixes #2608 (the manual is correct).

video/out/opengl/w32.c

@@ -338,7 +338,7 @@ static int w32_control(MPGLContext *ctx, int *events, int request, void *arg)
 }
 
 const struct mpgl_driver mpgl_driver_w32 = {
-    .name           = "w32",
+    .name           = "win",
     .priv_size      = sizeof(struct w32_context),
     .init           = w32_init,
     .reconfig       = w32_reconfig,