Update resolver.md

dependency -> dependencies
I checked its difference between https://doc.rust-lang.org/cargo/reference/resolver.html and here.

src/reference/resolver.md

@@ -248,7 +248,7 @@ situations:
   not currently being built. For example:
 
   ```toml
-  [dependency.common]
+  [dependencies.common]
   version = "1.0"
   features = ["f1"]