Change contribution_tools.md to refer to toolchain instead of explorer (#3499)

docs/project/contribution_tools.md

@@ -251,13 +251,13 @@ Pass `-c dbg` to `bazel build` in order to compile with debugging enabled. For
 example:
 
 ```shell
-bazel build -c dbg //explorer
+bazel build -c dbg //toolchain/driver:carbon
 ```
 
 Then debugging works with GDB:
 
 ```shell
-gdb bazel-bin/explorer/explorer
+gdb bazel-bin/toolchain/driver/carbon
 ```
 
 Note that LLVM uses DWARF v5 debug symbols, which means that GDB version 10.1 or
@@ -279,7 +279,7 @@ for more information. To workaround, provide the `--spawn_strategy=local` option
 to Bazel for the debug build, like:
 
 ```shell
-bazel build --spawn_strategy=local -c dbg //explorer
+bazel build --spawn_strategy=local -c dbg //toolchain/driver:carbon
 ```
 
 You should then be able to debug with `lldb`.