Added placeholder to camp interest email field

[cytadela8]

This change is 

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ab48142) 84.35% compared to head (a64a29a) 84.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #650 +/- ## ======================================= Coverage 84.35% 84.35% ======================================= Files 15 15 Lines 2512 2512 ======================================= Hits 2119 2119 Misses 393 393 ``` | [Impacted Files](https://codecov.io/gh/warsztatywww/aplikacjawww/pull/650?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=warsztatywww) | Coverage Δ | | |---|---|---| | [wwwapp/forms.py](https://codecov.io/gh/warsztatywww/aplikacjawww/pull/650?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=warsztatywww#diff-d3d3YXBwL2Zvcm1zLnB5) | `96.36% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=warsztatywww). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=warsztatywww)

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