[Readme] delete 'get_value' function and remplace it to 'get_string_value'

README.md

@@ -14,9 +14,13 @@ Think to end the address with a '/'.
 from nanospace import Nanospace
 
 nanospace = Nanospace('http://yourSrvUrl/','yourUsername', 'yourPassword')
-print(nanospace.get_value(66))
+print(nanospace.get_string_value(66))
 nanospace.update_value(66, 'sma', '9000000')
-print(nanospace.get_value(66)) # 9000000
+print(nanospace.get_string_value(66)) # 9000000
+
+print(nanospace.get_formula_value(66))
+nanospace.update_value(77, 'sma', '9000000')
+print(nanospace.get_formula_value(66)) # 9000000
 ```
 
 ### Creation of element