Doc: Fix broken \keyword link

Fix 19 counts of "Can't link to 'default-constructed value'"
warnings.

There was only one link to "default-constructed values", so the
cleanest solution was to change the keyword.

Also included a related typo fix.

Change-Id: I84b47743ecb72a1b2731ef65080fa1e630d478a4
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/doc/src/containers.qdoc

@@ -221,7 +221,7 @@
     \codeline
     \snippet streaming/main.cpp 2
 
-    \keyword default-constructed values
+    \keyword default-constructed value
 
     The documentation of certain container class functions refer to
     \e{default-constructed values}; for example, QVector

src/corelib/kernel/qvariant.cpp

@@ -2958,7 +2958,7 @@ QDebug operator<<(QDebug dbg, const QVariant::Type p)
 
     Returns the stored value converted to the template type \c{T}.
     Call canConvert() to find out whether a type can be converted.
-    If the value cannot be converted, \l{default-constructed value}
+    If the value cannot be converted, a \l{default-constructed value}
     will be returned.
 
     If the type \c{T} is supported by QVariant, this function behaves

src/corelib/tools/qpair.qdoc

@@ -84,7 +84,7 @@
 /*! \fn QPair::QPair()
 
     Constructs an empty pair. The \c first and \c second elements are
-    initialized with \l{default-constructed values}.
+    initialized with \l{default-constructed value}s.
 */
 
 /*!