[libc++] Fix off-by-one error in compare-benchmarks script

That led us to overwrite the data of the last row with the geomean.

Author: ldionne

libcxx/utils/compare-benchmarks

@@ -72,7 +72,7 @@ def plain_text_comparison(data, metric, baseline_name=None, candidate_name=None)
     geomean_0 = statistics.geometric_mean(data[f'{metric}_0'].dropna())
     geomean_1 = statistics.geometric_mean(data[f'{metric}_1'].dropna())
     geomean_row = ['Geomean', geomean_0, geomean_1, (geomean_1 - geomean_0), (geomean_1 - geomean_0) / geomean_0]
-    table.loc[table.index.max()] = geomean_row
+    table.loc[table.index.max() + 1] = geomean_row
 
     return tabulate.tabulate(table.set_index('benchmark'), headers=headers, floatfmt=fmt, numalign='right')