Eliminate accidental trigraph in FBString

Summary: `<::` is a trigraph, which, although removed in C++17, MSVC still warns about the sequences (although it doesn't actually intepret them as trigraphs)

Reviewed By: yfeldblum

Differential Revision: D3704382

fbshipit-source-id: b093f854343db01bc166b1cc44763676fdd97a51

folly/FBString.h

@@ -2441,7 +2441,7 @@ _GLIBCXX_END_NAMESPACE_VERSION
 
 #define FOLLY_FBSTRING_HASH1(T)                                        \
   template <>                                                          \
-  struct hash<::folly::basic_fbstring<T>> {                            \
+  struct hash< ::folly::basic_fbstring<T>> {                            \
     size_t operator()(const ::folly::basic_fbstring<T>& s) const {     \
       return ::folly::hash::fnv32_buf(s.data(), s.size() * sizeof(T)); \
     }                                                                  \