[libc] Fix quotation marks in overlay mode instructions

[DavidSpickett]

Summary

These were using ” ("right double quotation mark") instead of
the usual ". This means CMake took the value literally and you
got:

CMake Error at CMakeLists.txt:139 (MESSAGE):
  ”libc” isn't a known project:
  bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl;flang.
  Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?

="libc" works fine.

libc/docs/overlay_mode.rst

 We can treat the ``libc`` project as any other normal LLVM project and perform
 the CMake configure step as follows:
 
 .. code-block:: sh
 
   $> cd llvm-project  # The llvm-project checkout
   $> mkdir build
   $> cd build
-  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS=”libc”  \
+  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS="libc"  \
      -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
      -DCMAKE_BUILD_TYPE=<Debug|Release>                    \  # Select build type
      -DCMAKE_INSTALL_PREFIX=<Your prefix of choice>           # Optional
 
 Next, build the libc:
 
 .. code-block:: sh
 

 libcxx-abi, libc etc. are built using the ToT clang. The idea is that this build
 produces an in-sync toolchain of compiler + runtime libraries. Such a synchrony
 is not essential for the libc but can one still build the overlay static archive
 as part of the bootstrap build if one wants to. The first step is to configure
 appropriately:
 
 .. code-block:: sh
 
-  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS=”clang” \
-     -DLLVM_ENABLE_RUNTIMES=”libc”  \  # libc is listed as runtime and not as a project
+  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS="clang" \
+     -DLLVM_ENABLE_RUNTIMES="libc"  \  # libc is listed as runtime and not as a project
      -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
      -DCMAKE_BUILD_TYPE=<Debug|Release>                    \  # Select build type
      -DCMAKE_INSTALL_PREFIX=<Your prefix of choice>           # Optional
 
 The build and install steps are similar to the those used when configured
 as a normal project. Note that the build step takes much longer this time
 as ``clang`` will be built before building ``libllvmlibc.a``.