Disable a portion of one doc-test

* test/src/doc-tests.el (doc-test-substitute-command-keys):
Disable component that fails twice in the past 5 weeks
due to changes in the minibuffer map.
1 job for master in 58 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #4175


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1