Update slash_in_parameter.rst

"name" in requirements is confusing, I had problem for doing the same and then after 2 days I found that the "name" should be same as the parameter, it is not part of symfony config

   requirements: 
        name: .+
Changed userName to username
minor #4400 Continues #4307 (SamanShafigh, WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Continues #4307

Continues #4307

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -

Original PR description:

 > "name" in requirements is confusing, I had problem for doing the same and then after 2 days I found that the "name" should be same as the parameter, it is not part of symfony config

Commits
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b412780 Changed userName to username
48f453d Update slash_in_parameter.rst