Fix samples urls in README

[kacperfronc]

What is the purpose of the change

URLs in readmes pointing to samples are broken.

Brief changelog

Simple fix to 4 urls.

Verifying this change

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[codecov-io]

Codecov Report

Merging #6949 (8e14f97) into master (3fc2d10) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6949      +/-   ##
============================================
- Coverage     59.44%   59.36%   -0.08%     
+ Complexity      511      508       -3     
============================================
  Files          1028     1028              
  Lines         41519    41519              
  Branches       6037     6037              
============================================
- Hits          24679    24647      -32     
- Misses        14104    14123      +19     
- Partials       2736     2749      +13     

Impacted Files	Coverage Δ	Complexity Δ
...ng/transport/dispatcher/all/AllChannelHandler.java	82.75% <0.00%> (-6.90%)	0.00% <0.00%> (ø%)
...e/dubbo/remoting/transport/netty/NettyChannel.java	55.68% <0.00%> (-4.55%)	20.00% <0.00%> (-1.00%)
...mmon/serialize/kryo/utils/AbstractKryoFactory.java	75.43% <0.00%> (-3.51%)	0.00% <0.00%> (ø%)
...g/apache/dubbo/rpc/protocol/rest/RestProtocol.java	68.21% <0.00%> (-3.11%)	0.00% <0.00%> (ø%)
...g/apache/dubbo/config/AbstractInterfaceConfig.java	85.18% <0.00%> (-2.47%)	0.00% <0.00%> (ø%)
...he/dubbo/rpc/protocol/hessian/HessianProtocol.java	67.46% <0.00%> (-2.41%)	9.00% <0.00%> (ø%)
...n/java/org/apache/dubbo/config/RegistryConfig.java	84.12% <0.00%> (-2.39%)	0.00% <0.00%> (ø%)
...pache/dubbo/registry/support/AbstractRegistry.java	79.25% <0.00%> (-1.86%)	0.00% <0.00%> (ø%)
...ache/dubbo/cache/support/expiring/ExpiringMap.java	40.35% <0.00%> (-1.76%)	0.00% <0.00%> (ø%)
...che/dubbo/remoting/transport/mina/MinaChannel.java	50.00% <0.00%> (-1.32%)	16.00% <0.00%> (-1.00%)
... and 6 more

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[htynkn]

thanks for your contribution