Replace "identity transform" with "identity matrix"

[annevk]

The three times we say "identity transform" we mean "identity matrix".

To fix this search for the first string in the source resource and replace it with the second.

[domenic]

FWIW in mathematics I recall these being used fairly interchangeably; i.e. transforms are matrices. But I can see how it would be clearer to make the substitution here, so I agree this is a good change to make.