message: Fix "not constraint" grammar in docs
diff: @@ -7624,7 +7624,7 @@ class basic_json
  [RFC 7159](http://rfc7159.net/rfc7159) specifies:
  > An implementation may set limits on the maximum depth of nesting.
  - In this class, the object's limit of nesting is not constraint explicitly.
  + In this class, the object's limit of nesting is not explicitly constrained.
  However, a maximum depth of nesting may be introduced by the compiler or
  runtime environment. A theoretical limit can be queried by calling the
  @ref max_size function of a JSON object.
  @@ -7683,7 +7683,7 @@ class basic_json
  [RFC 7159](http://rfc7159.net/rfc7159) specifies:
  > An implementation may set limits on the maximum depth of nesting.
  - In this class, the array's limit of nesting is not constraint explicitly.
  + In this class, the array's limit of nesting is not explicitly constrained.
  However, a maximum depth of nesting may be introduced by the compiler or
  runtime environment. A theoretical limit can be queried by calling the
  @ref max_size function of a JSON array.
  