[lldb][docs] Update instructions to build standalone

The instructions to build LLDB standalone contain a CMake configure step
to build LLVM standalone. This configure step needs to also have the
CMake build type in order to work.

Author: chelcassanova

lldb/docs/resources/build.rst

@@ -210,6 +210,7 @@ Clang. Then we build the ``ALL`` target with ninja:
 ::
 
   $ cmake -B /path/to/llvm-build -G Ninja \
+          -DCMAKE_BUILD_TYPE=[<build type>] \
           -DLLVM_ENABLE_PROJECTS=clang \
           [<more cmake options>] /path/to/llvm-project/llvm
   $ ninja