Made it clear that this requires "natural" log

Author: ndattani

README.rst

@@ -226,7 +226,7 @@ Known problems
     Schwarz inequality to estimate upper limit of an integral, the default
     integral cutoff might not be accurate enough. It can be adjusted by the
     parameter ``env[PTR_EXPCUTOFF]`` (since libcint 4.0). This parameter needs to be
-    set to ``abs(log(cutoff_threshold))``.
+    set to ``abs(ln(cutoff_threshold))``.
 
 * On 64-bit systems, ``make test`` stop with error: ::