Fix a minor grammar glitch in new string / byte-array doc note

Amends 8845caa057cfcf54f7d46621adb3393c13747ffb.

Change-Id: I4bf09b9c1fff52815de58070fbe4ff0c08eff53f
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
 
 
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qbytearray.cpp

@@ -1439,7 +1439,7 @@ QByteArray &QByteArray::operator=(const char *str)
 
     \note Before Qt 5.14 it was possible to use this operator to access
     a character at an out-of-bounds position in the byte array, and
-    then assign to such position, causing the byte array to be
+    then assign to such a position, causing the byte array to be
     automatically resized. Furthermore, assigning a value to the
     returned QByteRef would cause a detach of the byte array, even if the
     byte array has been copied in the meanwhile (and the QByteRef kept

src/corelib/tools/qstring.cpp

@@ -5800,7 +5800,7 @@ QString QString::trimmed_helper(QString &str)
 
     \note Before Qt 5.14 it was possible to use this operator to access
     a character at an out-of-bounds position in the string, and
-    then assign to such position, causing the string to be
+    then assign to such a position, causing the string to be
     automatically resized. Furthermore, assigning a value to the
     returned QCharRef would cause a detach of the string, even if the
     string has been copied in the meanwhile (and the QCharRef kept