Change ernie accuracy results in README.md document (#22668)

test=release/1.7 test=document_fix

Author: lidanqing-intel

python/paddle/fluid/contrib/slim/tests/QAT_mkldnn_int8_readme.md

@@ -83,7 +83,7 @@ Notes:
 
 |     Model    |  FP32 Accuracy | QAT INT8 Accuracy | Accuracy Diff |
 |:------------:|:----------------------:|:----------------------:|:---------:|
-|   Ernie      |      0.80              |         0.82           |     +0.02 |               
+|   Ernie      |      79.76%            |         79.28%         |  -0.48%   |
 
 
 >**V. Ernie QAT2.0 MKL-DNN Performance on Intel(R) Xeon(R) Gold 6271**