Fix public target name of the random library (#684)

The name seems to be `random_random` rather than just `random`:

https://github.com/abseil/abseil-cpp/blob/master/absl/random/CMakeLists.txt#L19

CMake/README.md

@@ -93,7 +93,7 @@ absl::flags
 absl::memory
 absl::memory
 absl::meta
 absl::meta
 absl::numeric
 absl::numeric
-absl::random
+absl::random_random
 absl::strings
 absl::strings
 absl::synchronization
 absl::synchronization
 absl::time
 absl::time