updating the interval when doing bisectBracket()

to avoid getting stuck and iterate for no purpose. It might not affect the
result much, while from the code, it looks like it should be updated
iteratively.

opm/simulators/wells/WellBhpThpCalculator.cpp

@@ -695,6 +695,7 @@ bisectBracket(const std::function<double(const double)>& eq,
                 eq_high = eq(high);
                 abs_high = std::fabs(eq_high);
             }
+            interval = high - low;
             ++bracket_attempts;
         }