Doc: use standard wording in \brief commands

QDoc does some magic with the \class and \namespaces
and \brief commands, so the following wording must be used:
"The xxx class yyy ..."

Change-Id: Id231f30e8464898b776888d5423523de404aae34
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/projectexplorer/abiwidget.cpp

@@ -37,7 +37,7 @@
 /*!
     \class ProjectExplorer::AbiWidget
 
-    \brief A widget to set an ABI.
+    \brief The AbiWidget class is a widget to set an ABI.
 
     \sa ProjectExplorer::Abi
 */