edit dataset section

Added text to the dataset section.

docs/eml-semantic-annotations-primer.md

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    * URIs should be resolvable
    * annotations may be made at the dataset-level, entity-level attribute-level, in `/eml/annotations`, and in `/eml/additionalMetadata`
 
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 ## dataset-level annotation
 
+A dataset is defined as all of the information describing a data collection event. This event may take place over some period of time and include many actual collections (e.g. a time series or remote sensing application) or it could be just one actual collection (e.g. a day in the field). The `dataset` EML element encompasses all information about a single dataset. A dataset can be (and often is) composed of a series of data entities (see 'entity-level annotation' section below) that are linked together by particular integrity constraints. Further information about datasets may be found at:[insert link].
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 - describe that this annotation is embedded within an EML `dataset` element and the subject is the `dataset` id attribute value
 - describe the propertyURI and valueURI elements in the annotation, the label attributes of these elements, and the content of these elements  
 
-Example `dataset` annotation:
+Example 1: dataset-level annotation
 
 ```
 <dataset id="dataset-01">