VariableManager: Fix documentation

${var} has been replaced with %{var} since 2.2

Change-Id: Ifecd94fb68b8f7dfb4b4f56736d8c317cd6d41a2
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/plugins/coreplugin/variablemanager.cpp

@@ -78,7 +78,7 @@ public:
     Examples of this are \c{CurrentDocument:FilePath} and \c{CurrentDocument:Selection}.
 
     When the variable manager is requested to replace variables in a string, it looks for
-    variable names enclosed in ${ and }, like ${CurrentDocument:FilePath}.
+    variable names enclosed in %{ and }, like %{CurrentDocument:FilePath}.
 
     \note The names of the variables are stored as QByteArray. They are typically
     7-bit-clean. In cases where this is not possible, UTF-8 encoding is