Fix typos, noticed by Clang.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@113061 91177308-0d34-0410-b5e6-96231b3b80d8

src/chrono.cpp

@@ -94,7 +94,7 @@ init_monotonic_clock()
     return &monotonic_full;
 }
 
-#pragma GCC visiblity pop
+#pragma GCC visibility pop
 
 monotonic_clock::time_point
 monotonic_clock::now()

src/stdexcept.cpp

@@ -81,7 +81,7 @@ __libcpp_nmstr::~__libcpp_nmstr()
 
 }
 
-#pragma GCC visiblity pop
+#pragma GCC visibility pop
 
 namespace std  // purposefully not using versioning namespace
 {