RF: adjust common-principles to state that columns RECOMMENDED to be …

…in single not plural

src/common-principles.md

@@ -481,6 +481,7 @@ with two exceptions:
 It is RECOMMENDED that the column names in the header of the TSV file are
 written in [`snake_case`](https://en.wikipedia.org/wiki/Snake_case) with the
 first letter in lower case (for example, `variable_name`, not `Variable_name`).
+It is RECOMMENDED that the column names are singular (for example, `variable_name`, not `variable_names`).
 Column names defined in the header MUST be separated with tabs as for the data contents.
 Furthermore, column names MUST NOT be blank (that is, an empty string) and MUST NOT
 be duplicated within a single TSV file.