Update CHANGELOG.md

Author: apparentlymart

CHANGELOG.md

@@ -1,6 +1,7 @@
 # 1.7.1 (Unreleased)
 
 * `cty`: The `Value.Multiply` and `Value.Modulo` functions now correctly propagate the floating point precision of the arguments, which avoids generating incorrect results for large integer operands. ([#75](https://github.com/zclconf/go-cty/pull/75))
+* `convert`: The `convert.MismatchMessage` function will now correctly identify mismatching attributes in objects, rather than misreporting attributes that are actually present and correct. ([#78](https://github.com/zclconf/go-cty/pull/78))
 
 # 1.7.0 (October 20, 2020)