[docs] TestingGuide.rst: Fix incorrect description

This patch fixes two incorrect descriptions in TestingGuide.rst.

1. test/lit.site.cfg --> test/lit.site.cfg.py
   After https://reviews.llvm.org/D37838 , the `test/lit.site.cfg` had been added a .py extension.
   So it should be `test/lit.site.cfg.py`.

2. $(LLVM_OBJ_ROOT)/$(BuildMode)/bin --> $(LLVM_OBJ_ROOT)/bin
   The current build system doesn't create a $(BuildMode) directory any more.
   So it should be removed.

Reviewed By: mehdi_amini, MaskRay

Differential Revision: https://reviews.llvm.org/D140780

Author: DamonFool

llvm/docs/TestingGuide.rst

@@ -206,7 +206,7 @@ Writing new regression tests
 
 The regression test structure is very simple, but does require some
 information to be set. This information is gathered via ``cmake``
-and is written to a file, ``test/lit.site.cfg`` in the build directory.
+and is written to a file, ``test/lit.site.cfg.py`` in the build directory.
 The ``llvm/test`` Makefile does this work for you.
 
 In order for the regression tests to work, each directory of tests must
@@ -235,7 +235,7 @@ as many RUN lines as needed.
 
 :program:`lit` performs substitution on each RUN line to replace LLVM tool names
 with the full paths to the executable built for each tool (in
-``$(LLVM_OBJ_ROOT)/$(BuildMode)/bin)``. This ensures that :program:`lit` does
+``$(LLVM_OBJ_ROOT)/bin``). This ensures that :program:`lit` does
 not invoke any stray LLVM tools in the user's path during testing.
 
 Each RUN line is executed on its own, distinct from other lines unless