cmTarget: Fix typo

'a ALIAS' -> 'an ALIAS'

Source/cmTarget.cxx

@@ -5614,7 +5614,7 @@ void cmTarget::ComputeLinkImplementation(const char* config,
         {
         e << "Target \"" << this->GetName() << "\" links to target \"" << item
           << "\" but the target was not found.  Perhaps a find_package() "
-          "call is missing for an IMPORTED target, or a ALIAS target is "
+          "call is missing for an IMPORTED target, or an ALIAS target is "
           "missing?";
         this->Makefile->GetCMakeInstance()->IssueMessage(messageType,
                                                       e.str(),

Tests/RunCMake/CMP0028/CMP0028-NEW-stderr.txt

 CMake Error at CMP0028-NEW.cmake:4 \(add_library\):
   Target "foo" links to target "External::Library" but the target was not
   found.  Perhaps a find_package\(\) call is missing for an IMPORTED target, or
-  a ALIAS target is missing\?
+  an ALIAS target is missing\?
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)

Tests/RunCMake/CMP0028/CMP0028-WARN-stderr.txt

@@ -5,7 +5,7 @@ CMake Warning \(dev\) at CMP0028-WARN.cmake:2 \(add_library\):
 
   Target "foo" links to target "External::Library" but the target was not
   found.  Perhaps a find_package\(\) call is missing for an IMPORTED target, or
-  a ALIAS target is missing\?
+  an ALIAS target is missing\?
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
 This warning is for project developers.  Use -Wno-dev to suppress it.