[libcxx] NFC. Correct wordings of _LIBCPP_ASSERT debug messages

Differential Revision: https://reviews.llvm.org/D102195

NOKEYCHECK=True
GitOrigin-RevId: 96100f15082679b2c75c7744b8eb4fc87fcf71f5
diff --git a/include/vector b/include/vector
index 7ab97fa..6ee377b 100644
--- a/include/vector
+++ b/include/vector
@@ -674,22 +674,22 @@
 
     _LIBCPP_INLINE_VISIBILITY reference       front() _NOEXCEPT
     {
-        _LIBCPP_ASSERT(!empty(), "front() called for empty vector");
+        _LIBCPP_ASSERT(!empty(), "front() called on an empty vector");
         return *this->__begin_;
     }
     _LIBCPP_INLINE_VISIBILITY const_reference front() const _NOEXCEPT
     {
-        _LIBCPP_ASSERT(!empty(), "front() called for empty vector");
+        _LIBCPP_ASSERT(!empty(), "front() called on an empty vector");
         return *this->__begin_;
     }
     _LIBCPP_INLINE_VISIBILITY reference       back() _NOEXCEPT
     {
-        _LIBCPP_ASSERT(!empty(), "back() called for empty vector");
+        _LIBCPP_ASSERT(!empty(), "back() called on an empty vector");
         return *(this->__end_ - 1);
     }
     _LIBCPP_INLINE_VISIBILITY const_reference back()  const _NOEXCEPT
     {
-        _LIBCPP_ASSERT(!empty(), "back() called for empty vector");
+        _LIBCPP_ASSERT(!empty(), "back() called on an empty vector");
         return *(this->__end_ - 1);
     }
 
@@ -1698,7 +1698,7 @@
 void
 vector<_Tp, _Allocator>::pop_back()
 {
-    _LIBCPP_ASSERT(!empty(), "vector::pop_back called for empty vector");
+    _LIBCPP_ASSERT(!empty(), "vector::pop_back called on an empty vector");
     this->__destruct_at_end(this->__end_ - 1);
 }