Update CHANGELOG.md

Author: Paddy

CHANGELOG.md

@@ -7,6 +7,7 @@ BUG FIXES
 * Removed excessive logging when ExpectNonEmptyPlan was successfully matched [GH-556]
 * Fixed bug where state from data sources, which can't be imported, would be surfaced during ImportStateVerify [GH-555]
 * Fixed bug that ignored ExpectError when testing state imports [GH-550]
+* Fixed bug that sometimes prevented TestStep numbers from appearing in error output [GH-557]
 
 # 2.0.1 (August 10, 2020)