Add explicit symbol name for isKResult function on Readme

[0xkohe]

This PR updates the definition of the isKResult function by explicitly specifying its symbol name using [symbol(isKResult)].
Previously, [symbol] was used without an argument, which could lead to an Error like

[Error] Compiler: Definition is missing function isKResult required for
strictness. Please either declare sort KResult or declare 'syntax Bool ::=
isKResult(K) [symbol(isKResult), function]'
  while executing phase "resolving heat and cool attributes" on sentence at
        Source(/app/lesson-14-a.k/lesson-14-a.k)
        Location(30,8,30,55)

[ehildenb]

@0xkohe did you find this when compiling the tutorial programs?

[0xkohe]

@ehildenb

I compiled this yesterday using Docker image kframework-k:ubuntu-noble-7.1.248 on Ubuntu 22.04.5 LTS.

To be more specific, I used the following Dockerfile.

FROM runtimeverificationinc/kframework-k:ubuntu-noble-7.1.248
RUN apt-get update && apt-get install -y vim gdb
RUN mkdir -p /root/.config/gdb && \
echo "set auto-load safe-path /" >> /root/.config/gdb/gdbinit