fix: use TIMESTAMP_SUB for TimstampSub operation (#40)

Incorrect SQL expression was specified for TimestampSub.

Author: hwhitney456

ibis_bigquery/compiler.py

@@ -419,7 +419,7 @@ def _formatter(translator, expr):
             'TIMESTAMP_ADD', {'h', 'm', 's', 'ms', 'us'}
         ),
         ops.TimestampSub: _timestamp_op(
-            'TIMESTAMP_DIFF', {'h', 'm', 's', 'ms', 'us'}
+            'TIMESTAMP_SUB', {'h', 'm', 's', 'ms', 'us'}
         ),
         ops.DateAdd: _timestamp_op('DATE_ADD', {'D', 'W', 'M', 'Q', 'Y'}),
         ops.DateSub: _timestamp_op('DATE_SUB', {'D', 'W', 'M', 'Q', 'Y'}),