[dubbo-parent] Fix demo code error in readme file

[alexlavigne]

correting the demo code in readme

What is the purpose of the change

correting the demo code in readme

Brief changelog

correting the demo code in readme

Verifying this change

Verified.

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[ralf0131]

LGTM, thanks for the pull request!

[ralf0131]

I am re-triggering the CI, because some of the UT are unstable under Java 10.

[codecov-io]

Codecov Report

Merging #2122 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2122      +/-   ##
============================================
- Coverage     54.19%   54.14%   -0.05%     
+ Complexity     5085     5080       -5     
============================================
  Files           559      559              
  Lines         24922    24922              
  Branches       4442     4442              
============================================
- Hits          13506    13494      -12     
- Misses         9380     9388       +8     
- Partials       2036     2040       +4

Impacted Files	Coverage Δ	Complexity Δ
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-8.48%)	12% <0%> (-1%)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	45.83% <0%> (-4.17%)	3% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	51.89% <0%> (-3.8%)	10% <0%> (-3%)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...bo/remoting/transport/netty/NettyCodecAdapter.java	53.12% <0%> (-1.57%)	3% <0%> (ø)
...ache/dubbo/remoting/transport/mina/MinaClient.java	56.92% <0%> (-1.54%)	8% <0%> (-1%)
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	88.88% <0%> (+5.55%)	7% <0%> (+1%)	⬆️

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