docs: Update CONTRIBUTING.md [ci skip]

CONTRIBUTING.md

@@ -345,32 +345,33 @@ development environment.
 as they are executed. It is useful to inspect compilation flags.
 
 If no target or directory is specified, everything in the current directory
-is built. For example, all Boost.GIL tests can be built and run using:
+is built. For example, **all** Boost.GIL tests can be built and run using:
 
 ```shell
 cd libs/gil
-../../b2
+b2
 ```
 
 Run core tests only specifying location of directory with tests:
 
 ```shell
 cd libs/gil
-../../b2 cxxstd=11 test/core
+b2 cxxstd=11 test/core
 ```
 
 Run all tests for selected extension (from Boost root directory, as alternative):
 
 ```shell
-./b2 cxxstd=11 libs/gil/test/io
-./b2 cxxstd=11 libs/gil/test/numeric
-./b2 cxxstd=11 libs/gil/test/toolbox
+b2 cxxstd=11 libs/gil/test/extension/dynamic_image
+b2 cxxstd=11 libs/gil/test/extension/io
+b2 cxxstd=11 libs/gil/test/extension/numeric
+b2 cxxstd=11 libs/gil/test/extension/toolbox
 ```
 
 Run I/O extension tests bundled in target called `simple`:
 
 ```shell
-./b2 cxxstd=11 libs/gil/test/io//simple
+b2 cxxstd=11 libs/gil/test/io//simple
 ```
 
 **TIP:** Pass `b2` feature `cxxstd=11,14,17,2a` to request compilation for