doc: fix misc. mislabeled code block info strings

doc/guides/investigating_native_memory_leak.md

@@ -366,7 +366,7 @@ line number:
 
 This new output shows us exactly where the leak is occurring in the file `hello.cc`:
 
-```C++
+```cpp
   6 void* malloc_holder = nullptr;
   7 napi_value Method(napi_env env, napi_callback_info info) {
   8   napi_status status;

doc/guides/writing-and-running-benchmarks.md

@@ -59,7 +59,7 @@ package managers or download it from <https://www.r-project.org/>.
 The R packages `ggplot2` and `plyr` are also used and can be installed using
 the R REPL.
 
-```R
+```console
 $ R
 install.packages("ggplot2")
 install.packages("plyr")
@@ -71,7 +71,7 @@ selected first, specify a mirror by adding in the repo parameter.
 If we used the "<http://cran.us.r-project.org>" mirror, it could look something
 like this:
 
-```R
+```r
 install.packages("ggplot2", repo="http://cran.us.r-project.org")
 ```
 

src/README.md

@@ -910,7 +910,7 @@ Node.js provides a few macros that behave similar to `assert()`:
 The `OnScopeLeave()` function can be used to run a piece of code when leaving
 the current C++ scope.
 
-```c++
+```cpp
 static void GetUserInfo(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
   uv_passwd_t pwd;