simplify doctest

Co-authored-by: Martin Rubey <[email protected]>

Author: xcaruso

src/sage/rings/function_field/drinfeld_modules/charzero_drinfeld_module.py

@@ -315,7 +315,7 @@ def logarithm(self, prec=Infinity, name='z'):
             sage: log[2^5]
             1/(T^62 + T^61 + T^59 + ... + T^8 + T^6 + T^5)
 
-        On the contrary, when ``prec`` is a finite number, all the
+        If ``prec`` is a finite number, all the
         required coefficients are computed at once::
 
             sage: phi.logarithm(prec=10)