Update metadata.md

@DougManuel  noticed some typos on a quick scan. Also in the ERD - Sample table has a typo which has 'ID' instead of 'sampleID'

metadata.md

@@ -537,9 +537,9 @@ Used for lookup values of all category based columns
 -   **Merging tables** : Merging tables into a wide table requires additional steps when a variable does not have an unique name (when the variable name appears in more than one table). For example, variables such as `dateTime`, `notes`, `description`, `type`, `version` and `ID` variables such as `sampleID` are used in several tables. Use the following approach:
 
     -   Variable that are not unique (they are in more than one table): add the table name to the variable by concatenate column names with `_`. e.g. `dateTime` from the `Sample` table becomes `Sample_dateTime`.
-    -   Variable that are unique (they in only one table in the entire OMD). No variable name changes are needed.
+    -   Variable that are unique (they are in only one table in the entire OMD). No variable name changes are needed.
 
--   **Derived, summary or transformed measure**: These measures are generated to summarize or transform one or more variables. Naming convention follows the same approach as naming variable and category names, except use a `_` when concatenating variable or category names.  Examples of derived measure the calculation of a mean mean value of one or more SARS-CoV-2 regions. Normalization and standardization are other examples of a transformed measure. Typically derived, summary or transform measures are not reported, rather the preferred reporting approach reporting the underlying measures. 
+-   **Derived, summary or transformed measure**: These measures are generated to summarize or transform one or more variables. Naming convention follows the same approach as naming variable and category names, except use a `_` when concatenating variable or category names.  Examples of a derived measure is the calculation of a mean value of one or more SARS-CoV-2 regions. Normalization and standardization are other examples of a transformed measure. Typically derived, summary or transformed measures are not reported, rather the preferred reporting approach is reporting the underlying individual measures. 
 
 -   **Date time**: YYYY-MM-DD HH:mm:ss (24 hour format, in UTC)
 
@@ -655,4 +655,4 @@ wide table format
 
 ``` {.markdown}
     covidN1covidN2_PPMV_meanNr = 2
-```
+```