Update README.md to use `dart pub` instead of `pub` (#2195)

diff --git a/webdev/README.md b/webdev/README.md
index ee54737..6040c62 100644
--- a/webdev/README.md
+++ b/webdev/README.md
@@ -21,7 +21,7 @@
 ["activated"][activating].
 
 ```console
-$ pub global activate webdev
+$ dart pub global activate webdev
 ```
 
 Learn more about activating and using packages [here][pub global].