Small bug fix in example for #236 and more.

[nao1215]

This PR is a modification for the sample code, not to the ShellSpec itself.
I fixed two bugs that are like typo. One of the modification is to Issue #236.
The other was marked and commented as a bug in the code.

After the modification, the test results are as follows.

• Before

$ pwd
/home/nao/Desktop/shellspec/contrib/demo
$ shellspec 
Running: /bin/sh [sh]
.F

Examples:
  1) example add() function performs addition
     When call add 2 3

     1.1) The output should eq 5

            expected: 5
                 got: 6

          # spec/demo_spec.sh:14

Finished in 0.07 seconds (user 0.04 seconds, sys 0.00 seconds)
2 examples, 1 failure


Failure examples / Errors: (Listed here affect your suite's status)

shellspec spec/demo_spec.sh:12 # 1) example add() function performs addition FAILED

• After

$ shellspec 
Running: /bin/sh [sh]
..

Finished in 0.03 seconds (user 0.02 seconds, sys 0.01 seconds)
2 examples, 0 failures