Replace apostrophes (U+0027) used in snippets for grave accent (U+0060)

Using apostrophes in the Markdown files causes bad rendering on html files.

technical/ObjectDeletion.md

@@ -14,17 +14,17 @@ is not tidy.
 
 There are two methods in the ViewProvider that deal with this situation.
 
-'''c++
+```c++
 bool canDelete(App::DocumentObject *obj)
-'''
+```
 is called when Std_Delete is asked to delete obj, BUT it is called for each of
 obj's parent in the InList.  This is where you would decide if you will allow
 one of your children can be deleted.  In effect, the is canDeleteMyChildObject().
 Return true to allow the child to be deleted, or false to prevent the deletion.
 
-'''c++
+```c++
 bool onDelete(const std::vector<std::string> & parms)
-'''
+```
 is also called when when Std_Delete is asked to delete a DocumentObject, but in
 this case it is the DocumentObject's ViewProvider that is called.  Return true
 to allow deletion of the DocumentObject, or false to prevent the deletion.

technical/PropertyChanges.md

@@ -15,7 +15,7 @@ Recommended reading:
 - When a Property's value changes, its PropertyContainer (eventually a DocumentObject) executes the onChanged method with the Property as a parameter.  This is where you will take appropriate action based on the new property value.
 - Be cautious when inserting logic into onChanged and execute as it is possible to create a loop.
 
-'''c++
+```c++
 void myFeature::execute()
 {
     MyProperty->setValue(foo());
@@ -26,7 +26,7 @@ void myFeature::onChanged(App::Property property)
         execute();
     }
 }
-'''
+```
 - The other interesting method involving property changes is mustExecute.   This is where you control whether or not your object participates in the next recompute cycle.