[libc++] Fix typo in std::midpoint

Reviewed By: mclow.lists

Differential Revision: https://reviews.llvm.org/D71525

Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 6e8659c351fe3a38ed23f5a5b24c1bc143ea00ed
diff --git a/include/numeric b/include/numeric
index 11a5641..5ceadc1 100644
--- a/include/numeric
+++ b/include/numeric
@@ -573,7 +573,7 @@
     return __fp_abs(__a) <= __hi && __fp_abs(__b) <= __hi ?  // typical case: overflow is impossible
       (__a + __b)/2 :                                        // always correctly rounded
       __fp_abs(__a) < __lo ? __a + __b/2 :                   // not safe to halve a
-      __fp_abs(__a) < __lo ? __a/2 + __b :                   // not safe to halve b
+      __fp_abs(__b) < __lo ? __a/2 + __b :                   // not safe to halve b
       __a/2 + __b/2;                                         // otherwise correctly rounded
 }