[mediacapture-screen-share] Mangled text in spec (#187)

eladalon1983 has just created a new issue for https://github.com/w3c/mediacapture-screen-share:

== Mangled text in spec ==
The text reads:
```
If CS contains a member whose name, failedConstraint specifies a constrainable property,
constraint, applicable to display surfaces, and whose value in turn is a dictionary containing a
```
This appears to be a copy/paste error.

Please view or discuss this issue at https://github.com/w3c/mediacapture-screen-share/issues/187 using your GitHub account


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