Fix execution examples in JSON MSET doc

[enjoy-binbin]

In fact, executing the example directly will return this error:

redis> flushall
OK
redis> JSON.MSET doc1 $ '{"a":2}' doc2 $.f.a '3' doc3 $ '{"f1": {"a":1}, "f2":{"a":2}}'
(error) ERR new objects must be created at the root

Add some JSON.SET in front of the example to avoid error and fix
the incorrect responses.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a27a0a7) 78.04% compared to head (aa086e7) 78.04%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1162   +/-   ##
=======================================
  Coverage   78.04%   78.04%           
=======================================
  Files          15       15           
  Lines        3885     3885           
=======================================
  Hits         3032     3032           
  Misses        853      853           

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