Update BUILD.bazel

adding rust_doc_test to ffi example

Author: sebas-zip

examples/ffi/rust_calling_c/example_2/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@rules_rust//rust:defs.bzl", "rust_doc", "rust_library", "rust_test")
+load("@rules_rust//rust:defs.bzl", "rust_doc", "rust_doc_test", "rust_library", "rust_test")
 
 package(default_visibility = ["//rust_calling_c/example_2:__subpackages__"])
 
@@ -24,6 +24,11 @@ rust_doc(
     crate = ":matrix",
 )
 
+rust_doc_test(
+    name = "matrix_doc_test",
+    crate = ":matrix",
+)
+
 ## Do the same as above, but with a dynamic c library.
 
 rust_library(