add definition for glob in quick intro vignette

I also adjust the relative position of " Special environments" and "Obtain an R environment", since `Environment::global_env()` is introduced in the former

Author: Zhuoer Dong

vignettes/Rcpp-quickref.Rmd

@@ -434,17 +434,18 @@ double zz = Rf_rnorm(0, 2);
 # Environment
 
 ```cpp
-// Obtain an R environment
-Environment stats("package:stats");
-Environment env( 2 ); // by position
-
 // Special environments
 Environment::Rcpp_namespace();
 Environment::base_env();
 Environment::base_namespace();
 Environment::global_env();
 Environment::empty_env();
 
+// Obtain an R environment
+Environment stats("package:stats");
+Environment env( 2 ); // by position
+Environment glob = Environment::global_env();
+
 // Extract function from specific environment
 Function rnorm = stats["rnorm"];