Remove repeated words from BuildingLibcxx.rst

Summary: Remove repeated words from docs.

Reviewers: phosek

Reviewed By: phosek

Subscribers: christof

Differential Revision: https://reviews.llvm.org/D60530

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358147 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BuildingLibcxx.rst

@@ -201,7 +201,7 @@ libc++ specific options
 
   **Default**: ``OFF``
 
-  Do not export any symbols from the static libc++ library. This is useful when
+  Do not export any symbols from the static libc++ library.
   This is useful when the static libc++ library is being linked into shared
   libraries that may be used in with other shared libraries that use different
   C++ library. We want to avoid avoid exporting any libc++ symbols in that case.