Type in README.md

[jonericcook]

• What is the issue you have?
  Found a typo
• Please describe the steps to reproduce the issue. Can you provide a small but working code example?
  View this link https://github.com/nlohmann/json#json-merge-patch
• What is the expected behavior?
  // a JSON value
  json j_document = R"({
  "a": "b",
  "c": {
  "d": "e",
  "f": "g"
  }
  })"_json;

// a patch
json j_patch = R"({
"a":"z",
"c": {
"f": null
}
})"_json;

// apply the patch
j_document.merge_patch(j_patch);
// {
// "a": "z",
// "c": {
// "d": "e"
// }
// }

• And what is the actual behavior instead?
  // a JSON value
  json j_document = R"({
  "a": "b",
  "c": {
  "d": "e",
  "f": "g"
  }
  })"_json;

// a patch
json j_patch = R"({
"a":"z",
"c": {
"f": null
}
})"_json;

// apply the patch
j_original.merge_patch(j_patch);
// {
// "a": "z",
// "c": {
// "d": "e"
// }
// }

• Which compiler and operating system are you using? Is it a supported compiler?
  N/A
• Did you use a released version of the library or the version from the develop branch?
  develop branch
• If you experience a compilation error: can you compile and run the unit tests?

[nickaein]

Thanks for reporting!