Fixed Document `cmake --build` to `cmake --build .` (#13449)

`cmake --build` This command needs to specify the directory.

```
Usage: cmake --build <dir>             [options] [-- [native-options]]
       cmake --build --preset <preset> [options] [-- [native-options]]
Options:
  <dir>          = Project binary directory to be built.
```

Closes #13449

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/13449 from xinzhanguo:main 770317b8ec
PiperOrigin-RevId: 553944712

cmake/README.md

@@ -16,8 +16,10 @@ By default, CMake will use whatever C++ version is the system default.  Since
 protobuf requires C++14 or newer, sometimes you will need to explicitly override
 this.  For example, the following:
 
-    cmake . -DCMAKE_CXX_STANDARD=14
-    cmake --build
+```
+cmake . -DCMAKE_CXX_STANDARD=14
+cmake --build .
+```
 
 will build protobuf using C++14 (see [CXX_STANDARD](https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html#prop_tgt:CXX_STANDARD){.external} for all available options).