docs(ref): Shift focus to resolver v3

We have the `resolver` field in their as subtle nod to encourage people
to explicitly set it in workspaces.  With `resolver = "3"` being out for
a couple releases (at the time this hits stable), it seems appropriate
for us to bump this value.

Author: epage

src/doc/src/reference/workspaces.md

@@ -67,7 +67,7 @@ you want to keep all the packages organized in separate directories.
 # [PROJECT_DIR]/Cargo.toml
 [workspace]
 members = ["hello_world"]
-resolver = "2"
+resolver = "3"
 ```
 
 ```toml