Fix copy and paste error in CMake manual test instructions (#1062)

Both 5 and 6 were titled the same, but 6 sub steps are correctly
for the clean target

Fixup for #1046

Author: jonahgraham

cmake/org.eclipse.cdt.cmake.ui.tests/manualTests/README.md

@@ -33,7 +33,7 @@ Note, the Build Settings tab settings are stored separately for Run mode and Deb
 #### 5.1) Enter a custom all target, such as `helloworld`, and run build
   Expected: the custom target is used in the cmake command line
 
-### 6) Build Settings specific all target:  
+### 6) Build Settings specific clean target:
 #### 6.1) Enter a custom clean target, such as `help`, and clean project
   Expected: the custom target is used in the cmake command line