doc: update README.md for proxy-mirror

[scaat]

What this PR does / why we need it:

Fix #1847

[membphis]

please take a look at the output of the CI:

.travis/openwhisk-utilities/scancode/scanCode.py --config .travis/ASF-Release.cfg ./
Scan detected 2 error(s) in 2 file(s):
Reading configuration file [.travis/ASF-Release.cfg]...
Scanning files starting at [./]...
Scan detected 2 error(s) in 2 file(s):
  [./README.md]:
      73: line has trailing whitespace.
  [./README_CN.md]:
      72: line has trailing whitespace.

[scaat]

files have been corrected.