Update independent shellcommand state stdout_line and stderr_line from unbounded to 1

[vanderpol]

Describe the bug
state elements should only have a maxOccurs of 1. This was accidentally copied from the shellcommand item, and should be fixed.

Replication Steps (if applicable)
Line 1120: <xsd:element name="stdout_line" type="oval-def:EntityStateAnySimpleType" minOccurs="0" maxOccurs="unbounded">
Line 1130: <xsd:element name="stderr_line" type="oval-def:EntityStateStringType" minOccurs="0" maxOccurs="unbounded">

[vanderpol]

Resolved with related PR, closing issue