Use " for json quotes in examples not “”

[robmosys]

Docs issue template

Instead of the directionless quote symbols in code " uses the open and closing quotes “” ( \u201C \u201D). Means copypaste of json fails.

Page link

https://os.mbed.com/docs/mbed-os/v6.15/program-setup/build-rules.html

Expected to find...

{
    "target_overrides" : {
        "*" : {
            "target.features_add" : ["EXPERIMENTAL_API"]
     }
  }
}			

Found

{
    “target_overrides” : {
        “*” : {
            “target.features_add” : [“EXPERIMENTAL_API”]
     }
  }
}