fix: replace SHOULD with MUST in last changes (cc @effigies)

Please revert if these should not be replaced.

src/common-principles.md

@@ -439,7 +439,7 @@ large numbers of rows MAY be saved as
 [compressed tabular files (with extension `.tsv.gz`)](#compressed-tabular-files),
 which are introduced below.
 Plain-text TSV and compressed TSV are not interchangeable, that is, each section
-of the specification prescribes which one SHOULD be used for the data type at
+of the specification prescribes which one MUST be used for the data type at
 hand.
 Each TSV file MUST start with a header line listing the names of all columns
 with two exceptions:
@@ -562,7 +562,7 @@ like in the example below.
 
 ### Compressed tabular files
 
-Large tabular information, such as physiological recordings, SHOULD be stored with
+Large tabular information, such as physiological recordings, MUST be stored with
 [compressed tab-delineated (TSVGZ) files](glossary.md#tsvgz-extensions) when
 so established by the specifications.
 Rules for formatting plain-text tabular files apply to TSVGZ files with three exceptions: