[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/iterator b/include/iterator
index 687c641..6bc98ec 100644
--- a/include/iterator
+++ b/include/iterator
@@ -1362,7 +1362,7 @@
     {
 #if _LIBCPP_DEBUG_LEVEL == 2
         _LIBCPP_ASSERT(__get_const_db()->__dereferenceable(this),
-                       "Attempted to increment non-incrementable iterator");
+                       "Attempted to increment a non-incrementable iterator");
 #endif
         ++__i;
         return *this;
@@ -1374,7 +1374,7 @@
     {
 #if _LIBCPP_DEBUG_LEVEL == 2
         _LIBCPP_ASSERT(__get_const_db()->__decrementable(this),
-                       "Attempted to decrement non-decrementable iterator");
+                       "Attempted to decrement a non-decrementable iterator");
 #endif
         --__i;
         return *this;
@@ -1387,7 +1387,7 @@
     {
 #if _LIBCPP_DEBUG_LEVEL == 2
         _LIBCPP_ASSERT(__get_const_db()->__addable(this, __n),
-                   "Attempted to add/subtract iterator outside of valid range");
+                   "Attempted to add/subtract an iterator outside its valid range");
 #endif
         __i += __n;
         return *this;
@@ -1400,7 +1400,7 @@
     {
 #if _LIBCPP_DEBUG_LEVEL == 2
         _LIBCPP_ASSERT(__get_const_db()->__subscriptable(this, __n),
-                   "Attempted to subscript iterator outside of valid range");
+                   "Attempted to subscript an iterator outside its valid range");
 #endif
         return __i[__n];
     }