update php paged map field snippets

[bshaffer]

Update the sample code for mapped fields to include the key for clarity.

[codecov]

Codecov Report

Merging #3325 (c2443d8) into master (7c508c8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #3325   +/-   ##
=========================================
  Coverage     87.13%   87.13%           
  Complexity     6120     6120           
=========================================
  Files           495      495           
  Lines         24173    24174    +1     
  Branches       2638     2638           
=========================================
+ Hits          21064    21065    +1     
  Misses         2242     2242           
  Partials        867      867           

Impacted Files	Coverage Δ	Complexity Δ
...codegen/viewmodel/PageStreamingDescriptorView.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
.../codegen/transformer/PageStreamingTransformer.java	100.00% <100.00%> (ø)	12.00 <0.00> (ø)

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[vam-google]

LGTM